How much would I benefit from an amp upgrade?

I owned the SC25 for a bit. It was probably the worst sounding piece of equipment I've ever owned. Powerful, detailed, artificial, digital, unmusical.

I switched to a NAD T765HD and all is well in the world.

Just my opinion.

I suppose you could experiment with different power amps, but it will be hit and miss.

I'm actually pretty happy with the receiver other than the fact that it didn't quite have the power for my fronts.
I put it in Stereo when listening to the turntable (direct into receiver using it's phono stage for now) and use surround modes for theater or 5.1 audio. Listening to Wish You Were Here Immersion Edition on BluRay this morning in 5.1 was wonderful! It breathed new life into tracks I've heard thousands of times.

I should probably try biamping the fronts to give them a theoretical 280W instead of a theoretical 140W but I'm wondering if I really have much to gain with a higher end amp vs. the Denon POA which to me has been great.

The KEF Reference 203.1's have 5 speakers to drive in each tower so extra power is good. FYI, they actually have binding posts to Tri-Amp.

So I guess my real question is, how good are the Denon POA-2200/2400/2800/1500 amps vs higher end models out now? Will I really notice the upgrade?

Denon POA-2400
System: transistor
Power consumption: 500 W @ 120/240 V

Power Amplifier
Speaker Outputs: 2 x 2 Speakers
Continuous Power 4Ω: 330 + 330 (DIN) Watts
Continuous power 8Ω: 200 + 200 Watts
Dynamic Power 4Ω: 450 + 450 Watts
Frequency Response: 1-100,000 +0/-3 dB (normal); 1-300,000 +0/-3 dB (DIRECT) HZ
Input Sensitivity: 1,000 (Normal/DIRECT) mV
Input Impedance: 25 (normal/DIRECT) kΩ
Signal to Noise Ratio: 123 (IHF A) db A, short circuited
Harmonic Distortion: < 0.002 (-3 dB/ 8 Ώ) % at rated output

Physical
Dimensions in Inches: 17.1 x 7.4 x 16.9 (w) x (h) x (d)
Dimensions in mm: 434 x 187 x 429 (w) x (h) x (d)
Weight in lbs: 38.6 lbs
Weight in Kg: 17.5 Kg

I did a ton of upgrades this year and in my experience the amp upgrade makes the smallest difference and the room treatments make by far the biggest.

Get the current Krell 300 series integrated; they list at $2500 and appear on here regularly for around $1700. Will sound MUCH better than the kind of amps you mention; not all watts are created equal. The 203 deserve a better quality amp; but don't forget the room treatments either. Just to check I took most of mine down last week and the sound was much worse.