I own the EL34 104 head, and have compared it to the 6L6 head through the same cabinet with the same guitar (my personal Rectifier 2x12 and Les Paul Traditional Pro). I've read this forum for a while, and finally have something I can contribute. Myself I am a guitarist of around 12 years, as a serious hobbyist/amateur.
Here are some clips I recorded of my 104 head today. I am not as good a guitarist or engineer/producer as most of the guys on this forum but I hope they are of help to some. With the metal demo I just went through some different styles/tunings showing boosted and unboosted (I prefer to boost my OD channels) and both channels, without touching the EQ. In the ISF demos I only move the ISF control to show its scope on the OD channels.
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Quick reviews of the 104 heads -
EL34 Pros - Broader range of usable tones, can get warm and smooth leads more easily. Can cover 'classic rock' territory more convincingly
Cons - OD1 and OD2 are harder to get to work together with the shared EQ compared to the 6L6. Awful knob colour, very difficult to see settings. Can get bright easiy. Not as punchy for palm mutes nor as aggressive for metal.
6L6 Pros - Arguably nicer clean (gets more glassy and fenderish), Super crunch channel has the best tone for metal out of both heads/all channels in my opinion, OD1 and OD2 shared EQ works better in my opinion. Much better knob colour which is a big pro for live. Punchier and more aggressive overall
Cons - Harder to get a warm sound of of this head, harder to get smooth fat leads, didn't find the OD channels as versitile as I couldn't dial out the aggression.
General Pros - quality switches and knobs, excellent series loop, good footswitch, DPR works as advertised, midi.
General Cons - Knob design not easiest to see (though way better on the 6L6), unbalanced due to transformer design, shared EQs, clean modes/crunch modes not footswitchable
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Long reviews - there are a lot of differences, and sadly not enough good demos showing them side by side at this time.
Clean Channels - To me both amps have a great clean which works fine with the shared EQ for the crunch in my opinion. The EL34 sounds more Marshall-ish, and 6L6 more Fender-ish regardless of ISF, especially in relation to the highs. If you like American Cleans the 6L6 does it better. Warm clean is the best option on both heads, I personally never use the bright option. Some have suggested this mode sounds AC30 ish, I own an AC30 and AC4 and disagree, though maybe through a different speaker cabinet results may be different.
Crunch channels - to me the 6L6 has more gain and sounds more aggressive on both crunch and super crunch. Personally the 6L6 super crunch is my favourite channel overall out of both amps for metal, it has enough gain but has a great core tone when compared to the OD channels, more on that later. The EL34 head crunch/super crunch is to my knowledge the same as in the regular 100 watt 2 channel, sounds a bit Marshall JCM and will serve you well if you want a more tweakable tone in that ballpark. As I mostly write and record non-metal, I use this channel the most. Here is an example of the Crunch channel with gain about 7/10, Mesa Rectifier 2x12 and same guitar as the videos
http://soundcloud.com/maxmoujaes/bends_s104_el34
OD channels - this is where I found the biggest differences. The 6L6 head gets quite metal here, and to me sounds very aggressive regardless of where the ISF control is, you just get different shades of angry. This might be exactly what you want. Also, I thought that the OD channels shared EQ made sense on the 6L6, with the differences between the channels being less severe than on the EL34 head. For live use and for metal this would be my preference, especially when you consider the knobs are easier to read and the Super Crunch channel is very good indeed. The bass response on this head was also tighter and punchier for palm mutes but please be aware I play in standard tuning on a Les Paul with a Bursbucker 3 (A2 pickup), so I was hardly testing the depths of the amp's response to an extended range guitar. I'd still say the OD channels sound best boosted on this head, it isn't as tight as a 5150 type of amp.
The EL34 head OD channels are a mixed bag of love and hate. OD1 is smooth on the presence and tight on the bass response (though not as tight as the 6L6 head) and lends itself really well to leads, or as a 'more gain and volume' channel to pair with the crunch channel. It sounds like a beefy version of channel 2 crunch. OD2 has a comparatively aggressive presence and a much looser feel, and feels like someone turned the afterburners on the Super Crunch channel from channel 2. To me the jump in presence and relative scoop in mids (due to the bass getting bigger and looser) means some shared settings don't work too well. For example if you like the tightness of OD1 and want to run it with a bright sound, switching to OD2 can bring way too much presence and make things too piercing. Likewise a smooth fat sound on OD2 may make OD1 too dull to use as a rhythm channel. With EQ settings in the middle I find the two can work together, however if you want a very fat or alternatively very bright tone out of one of the EL34 OD channels the amp becomes more of a 3 channel - you just use crunch/super crunch for your alternate gain tone. This is all my opinion of course, and for recording it doesn't really matter.
Some observations regarding both heads - like on many high gainers there is too much gain on tap. I'm only using passive gibson humbuckers and beyond about 4/10 unboosted on any of the OD channels is enough gain for most styles of metal. Personally I found the supercrunch channel to have a more complex and interesting tone when compared to the OD channels on both amps. The OD channels don't have as much character, but still sound good. This is something best experienced in person and is subjective. The DPR works very well, though you won't ever want to run it at less than 100 for metal rhythm as you lose to much bass response. The Effects loop is series and has +4 as well as -10 options, and works extremely well. The knobs and switches are very solid and high quality. The heads are heavy and unbalanced, with the transformers being on one side, but it was not a deal breaker for me.
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Other Blackstar stuff..
I also own the oversize straight S1 4x12, but can only comment on it for home use as I tend to take my rectifier 2x12 to jams for convenience. It is similar to a Rectifier 4x12 but a bit less boomy and not quite as dark. It is still boomy compared to a standard 4x12, but I liked how chewy I could get the mids and think the quality is high, no rattles at all which is more than I could say for my previous Line V30 6 4x12. I compared it to an Orange PPC412 at the time of purchase, and while the orange is smoother and punchier I found it too fat in the mids and too round in the highs on the OD channels. The orange sounded great on the crunch channels but I personally found the S1 oversize 4x12 more versatile, if a bit bright at times. It has mellowed out through months of use and I really like the cab. The wheels are not as large as mesa ones, but they do push in/pop out very easily.
I have previous experience with owning the HT1R, HT60 stage and HT100 head and HT2x12 cab, as well as the Artisan 2x12 cab... All those have been sold... I can compare the HT 3 channels to the S104 EL34 through the same cab (sorry no A/B clips, just personal experience) but this is not the right post to elaborate on and I have waffled on enough already.