Sorry for being late on replying this tread. Ok, I have some deduction for newbie to help in his decision. Please take note that these are my personal view, opinions and deductions, as an amp repairman. I try to be un-bias. Again I just like to share to all that I do not play the electric guitar, but I more or less can relate to whatever tone and effect you are refering to.
Tech21 10inch - Since you are looking for power low end sound, I suggest you look at a 12 inch model, more full range. I forgot if I have went thru a t21 amp before. But if it's modern suface mount PCB, it may give you a headache when it breaks down. The sound I can't comment since I haven't heard one yet.
Peavey Studio Pro 112 - Again, as much I dont like to, as I cannot deal in peavey products, I'd reccomend this amp. The modern transtube II peaveys are very tone versatile, you can dial in to actual analoge tone shaping on these amps. The purist will always say, keep it pure, and this design is still a pure analoge machine. Thus the tones it can produce is remarkable. Again, peavey stuffs are made reliable, even if these days they are also made in china, so is everything else.
Marshall AVT20 - The marshall tone says it all. Either you love it or you hate it. OK, your concern at low gain, you loose tone. THAT'S RIGHT, it has a 12AX7 pre-amp tube, so the sound will change if the input is low level. That's just tube. So use it as a gain machine, like we said, a marshall is not meant to be built for clean sound anyway. Choose this if you love the marshall tone.
Kustom 65- Cheap and gainy amps too. The KGA65 model was pretty popular with rockers. But remember it do spot digital effects on board. Either you love that or detest that feature. I think the plus point is the price, and it sport a Celestion driver, but remember not all Celestion equipped amps sound good, they produce cheapo stuffs too. Sound wise is average or slighty above.
Smarvo - Newcomer, New brand, chinese cheapo amps. I endorse these as I carry the product, so have to sell them lah. OK, I admit what you pay is what you get. For S$400 you dont get a second hand peavey studio pro 112 (new $520). For $400 YOU CAN GET A 2X12" 100WATTS STEREO CHORUS BIG AMP!!!! Hahaha....Most bang for the buck, IMO. The manufacturer also produce amps for very branded company (Not marshall, hint, hint). I think this amp will win hands-down in the low end sound to any of the above amps. The only disadvantage, it's BIG, advantage is that you can use it for gigs and performances.