OK man, I have had a 5150 combo for about 8 years and here is the scoop (I still have it and love it). After the 5150 Original head was made Eddie started to want modifications so the Peavey dudes started working on changes. This resulted in a head that had a much warmer tube bias (sweeter distortion and body) and more bottom end. Eventually this prototype was ditched for the 5150 II (which has the added gain stage and preamp tube among other things).
So to not let the prototype head go to waste they came up with the idea to put it in a combo. Unlike an ordinary combo a half of a half stack (quarter stack if you will). Basically it is the same quality and construction of a good 4x12 cab but chopped in half with a head built in.
Since the 5150 combos have a much warmer bias with the right tubes (see bob at eurotubes.com). They sound amazing even with out any modifications.
Your questions, answer 1: The speakers sound pretty close to a Mesa Cab loaded with v30s and you can replace the stock speakers with v30s. answer 2: yes this sucker will hook into a 2x12 (mesa has a nice 2x12), hell it can power 2 other 4x12s if you want. I run the stock speakers in the combo and haul this thing around to band practice. I have a Marshall 1960AV (with v30s) I use when tracking as a secondary tone. Basically I record one pass with the stock speakers and one pass with the 5150 combo into the Marshall cab. Yes it sounds like a well modded warmer punchier 5150 head into a 4x12 in that case.
Only issue is this bad ass is heavy (like in weight) but on the original models they only build one lame ass "practice amp" style handle onto the amp (mine broke in a week). I mean come on it is like a head and half of a half stack all in one cabinet it is heavy. Side Marshall style handles would be nice. I hear on the new 6505 combos they have added two more practice amp handles on the top of the amp near the sides.
