Image T6 spec sheet says 8 ohms nominal, 4 ohms min. That is actually not a bad load for an amp; many speakers dip below 4 ohms. Note THX certification requires the amp be able to drive 3.2 ohms.

Denon AVR-4311Ci = 140 W/ch at 8 ohms, 170 W/ch at 6 ohms (lowest nominal impedance rating though it doeslist 220 W "dynamic" power into 4 ohms, 2 channels -- at least it actually has something at 4 ohms!)

Emotiva XPA-5 = 200 W/ch at 8 ohms, 300 W/ch at 4 ohms

No apples-to-apples... Comparing a couple of the numbers:

140 W vs. 200 W => 1.55 dB

170 W vs. 300 W => 2.47 dB

A change of 1 dB is barely perceptible to most of us, 3 dB is just noticeable, and 10 dB is twice as loud (and takes ten times the power). From a volume standpoint, the extra headroom is very probably in the mud. Will the Emotiva drive the speakers "better"? Hard to say, but at normal volume levels I strongly suspect no difference would be heard.

OTOH, the 4311 adds Audyssey XT32, a significant advance in the ability to correct in-room frequency response, something that trumps the amp argument IMO. That is, I would expect, for this system, far more improvement from XT32 than from adding an amp.

One counter: if the VSX-50 really is underpowered, something I tend to doubt, then the same argument applies in reverse. The VSX-50 is rated at 90 W/ch, so power-wise the step-up to the 4311 is not terribly significant (from 90 W to 140 W is only 1.92 dB). Perhaps the OP should check one of the many in-room SPL calculators to see how much power he might need to hit say 100 or 105 dB. One is

http://myhometheater.homestead.com/splcalculator.html
YMMV, IMO, FWIWFM, etc. - Don