I Have an Alpine MRV-T407 amp. I've heard that Alpine underrates their amps. Is that correct? I'm wanting to get 4 Full range speakers (two rear and Two Fronts) that handle 100 watts apiece and i was trying to detirmine if I should get another amp of if the one I have can handle it.

Try a 4 channel amp maybe??? As for Alpine amps, I'm not really sure if they are underrated but they perform well. I have had three Alpine amps and they were great.

Yes, Going on Benchmarks (search some car audio forums) they are slightly underrated...

Also Does anybody know if a 61/2 inch speaker with a 2 15/16" mounting depth would fit. I was a little bit skeptical about the depth. Some of the pioneer 4x6s are that deep too so if anybody has ever tried one similar....

that should be ifne. 6 1/2 def fits, and that depth should work. What speakers are you using that are that deep..

A Pair of Alpine SPR-17C 6-1/2's. Specs say they'll handle 100W RMS. I can buy a pair for $89.00. The only problem is my amp runs 70w x2 channels rms and I wondered If I'd need another amp to support the power needs of my speakers. By the way which is more important. the top mount depth of the bottom mount depth. Cuz the Bottom-mount depth is 3-5/16".

worry about top mount, thats what youll be using...

They will fit fine, and the amp is fine. 70w will be ok on them