Amolet
First - M3Tis don't get the respect they deserve around here. They are GREAT speakers. That having been said,
I have a Sony STRDB 1070 rated at 100 wpc x 6, and figure it never put out more than 30 wpc, but has an excellent front end and if you put some decent amplification behind it, can sound great. So, here are a few suggestions:
Option 1
Keep Sony as processor
Keep the M3Tis
Buy used 5 channel amp. Graduating or flunking out of state students should be selling some equpt for cheap. Don't pay more than $350. Nice Adcom, Integra, Rotel, or you can find 3 used stereo amps.
Get a pair of M2s for your side position speakers from FO ($230) You can always move these to the back position when you go 7.1 and then get a pair of QS8s for the side position.
partsexpress.com Vance Dickason Titanic 10" subwoofer. $350 delivered.
Sell your subwoofer ($100?)
You have $170 change under option 1.
Option 2
Same as above, except buy QS8s instead of the M2s. You are now about $25 over your limit
Option 3.
Sell the Sony for $150-200. Put money in the pocket.
Sell your Sub put another $100-150 in the pocket.
Buy Panasonic XR45 digitally amplified receiver from internet, do not pay more than $300, or get Panasonic SAHE 200 receiver for the same price. The former does not have preouts but is reported to sound great, incredibly low noise floor, once broken in, very extended highs and terrific mids. The latter is just your basic Panasonic overachieving 6.1 receiver which I like better than the Yamaha 1400, much better than the sub $1000 HKs.
Trade in your VP100 for a VP150, or sell the VP100 and buy the VP150.
At this point, depending on how good a salesperson you are, the Panasonic should have cost you a net $0, and the VP150 shouldn't have cost you more than $200 after you sold the VP100s. (don't know how much you get in trade from Axiom)
So you're sitting with a new giant killing receiver and one of the finest center channels available and should be no more than $200 out of pocket.
Buy a pair of M2s for the side position. ($230 FO)
You now have $570 to spend on a subwoofer. You could buy 2 used Velodyne CHT 12" subs and use them as snack tables as well as low frequency sonic violence generators, or you could check out used subs locally.
I think Option 3 looks better.
Option 4
Read the cases, think about the law, don't watch movies on your system, listen to music which helps you study.
In any case, good luck!! Enjoy the music.
