Hi Chimaera,
Are you using the Sonic Impact T-Amp? I think you mentioned a while back that you are.
I'm not sure why the sound has degraded over time... could be a defective amp?

Luckily, it is only around $30 or $40 to replace

I'm really itching for a Sig 30 (with Visaton b200s of course) but being a student its a pipe-dream at the moment lol. I wonder what should I upgrade next?
I would
first upgrade your speakers. Get the best that you can afford, and continue to use the T-Amp (but make sure the speakers are at least 90dB efficient with that 6-watt RMS into 8-ohms per channel T-Amp) until you later have the funds to upgrade to a better amp. The Signature 30 will crush the T-Amp (not just in terms of having 4 times the power, but in terms of the sound quality). Still, the SI T-Amp is priced extremely well and is good to start with.
Oh and for what its worth I do find the T-amp tiring to listen to during a longer session, does that happen with a Sig 30 at all?
It could be your speakers, the T-amp, your source, or a combination of any of them. I have a hunch that your speakers are what needs to be upgraded first.
I'm only drawing comparisons because they are both tripath-based amps and I haven't heard anything else other than my T-amp so far.
Amplifiers that use Tripath chips do not all sound the same... the sound can vary a lot! There is more to the design than the Tripath chip. There are input stage components, output stage components, board layout, power supply, volume control, etc. All of this will make or break the sound.
Also, the Signature 30 is 30W RMS into 8-ohm per channel. That is more than 4 times the power than the SI T-Amp. If you are clipping your speakers, you might need more power (unless it is your speakers that can't handle the power you are feeding them and are breaking up, but I bet it is the low power of your amp). However, having 4 times the power does not equal to 4 times the volume
Hope this helps,
Vinnie