How much would I benefit from an amp upgrade?

I am a big believer in 2-channel for 2-channel, it makes a very big difference. There are some great integrated amps with home theater pass-through which will serve your needs well. I have not heard the Krell 300i but that is on the right track.

I think you'd benefit a lot with a better amp. Right now there are a couple of new and demo Theta Intrepid 5 channel amps on Audiogon going for around $1,400. IMO, that's a no-brainer.

i agree regarding the krell s300i. can be found dirt cheap considering the power and HT features. had one for a while and enjoyed it. give one a try if you can. easy to flip used units if things don't work out.

My experience is in going from a BAT power amp for two channel to a NAD 763 paired with a M15 prepro for video. I kind of got out of Vinyl for a while but recently replaced the M15 with a NAD T175HD in order to be able to decode lossless codecs and 96k. The surprise for me was I think the two channel sound (which I thought would get worse) is actually better, perhaps due to the ability to use Audussy in a difficult room. I think I am getting the best sound ever from my vinyl now and I have been in this room for 20+ years and at one time had Appogees and a Krell.

Long story short, while I am probably giving up a bit to the BAT I am very happy with the sound. While not directly related to your equipment the point is that some relatively inexpensive power amps stack up quite well against some well regarded expensive equipment. On the other hand this rig is head and shoulders above a Yamaha receiver I used for a while when I first went into theater.

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.

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.

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 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.