Amp upgrade worth it for Sierra 2EX?

I'm currently running a 5.1.4 setup with a Denon X4400 and Sierra 2EX's for my LCR. My surround are my old, repurposed Energy speakers. I want to expand to 7.1.4, using 2 more leftover Energy speakers.

The X4400 processes 11 channels, but only has 9 amp channels. So, I can purchase a cheap 2 channel amp and use it for the top rear surrounds (the least used speakers), or I can go the more expensive route of buying a better 2 channel amp, and using it for the front Left and Right Sierra 2EXs, opening up those internal amp channels to be used for the additional surrounds.

My question is, how much do I need to spend on an amp to improve on the sound quality of the internal amps in the X4400 to make it worthwhile? Someone on another forum suggested something like the $350-ish Crown XLS1002. I'm not sure if that is an improvement in that price range, or if it's basically the same thing.

Would using something like that to power the Left and Right Sierra 2EXs be an improvement over the internal amps? Or is it something that's not really going to make a noticeable difference unless I am willing to spend a higher dollar amount on a better 2 channel amp.

If it's not going to really make a difference, then I'll just grab a $125 2 channel amp for the extra 2 surrounds and call it a day.

I clearly preferred a separate amp to my Denon 6200 for my fronts with good music. I think you'd get the most (best chance for a benefit) from 2-3 channels (maybe even as two stereo amps) of "good" amplification and use it up front. Used will get you the best bang for the buck. Similarly, you could go with a good used HT amp, with even more 'good' channels.

I don't think you're going to see noticeable improvements.

I have a Marantz SR6013 which I thought had good sound but I purchased a Cambridge Audio AXR85 and was blown away with the 2-channel sound. I eventually upgraded to the Cambridge Audio CXA81 which was an even bigger improvement. I switched between the 2 amp and my Marantz for movies only.

I think a good 2-channel amp is better for music than an HT amp.