nothing wrong with the jcm2000 line. the other guitar player in my band uses the tsl100, sounds decent. similar to the jmp1/ el34 100:100 setup. the trick for a tight metal rhythm sound is to use the crunch channel for rhythm. the lead channel indeed is kinda sloppy and harsh, so i can see why most guys (who likely skip the crunch channel for metal) say it's crap.
as for the dsl100, i used one for 20 shows during an euro tour. for a slayer meets the haunted like rhythm tone, use high output pickups, the lower gain mode of the lead channel, and don't turn the gain past 6-7. lets see if i can recall my tour settings....
presence was almost off, like 1 or something, just enough to give it a certain sizzle and edge. bass around 8, mids halfway up, treble anywhere between 3 and 5, gain around 6 1/2, tone shift and deep off, master around 4-5 depending on the venue. i mostly used an emg81 loaded jackson straight in, going through a marshall 1960 a/b fullstack. i kinda wish i'd own that setup, actually i can't recall how often i wished to own a jcm2000.
the tsl100 crunch channel is a bit darker, that's basically the whole difference imho. keep in mind, that's not very saturated, so all the mistakes will shine through. i keep my gain pretty low anyways, so i'm used to it, and therefore i assume that sound will work just fine in a recording session.
i'd use it for anything up to speed/thrash metal....anything heavier than that (swede death, death, black, grind) go get something different. although it might get there with a good stomp in front, as i said i never tried that, went straight in all the time.
if you need to go heavier than that, check out the 5150. it can get kinda dsl-esque using the crunch channel (gain around noon) boosted with an sd1 to keep the lows in check and to add some bite, plus you got the lead channel for a lead boost or the ultra heavy stuff. in general, the dsl is gonna be darker and more midrangey, it has that classic marshall midrange grind which you either love or hate - i love it.
sorry for the long post. i just cannot understand all those "dsl/tsl sucks donkey balls" comments. they might not be my first choice, esp. as they're a bit overpriced IMHO (a brand new dsl is like 1100 euro iirc, i'd not pay any more than 800), but you can definitely get them to sound decent without much effort - depending on the sound you're going for