i have a question about a amplifier im working on
I'm not sure if there's a question in there anywhere, but here are a few points of explanation:


The voltage rating on the transformer is an RMS value. 40 VRMS corresponds to a rectified peak value of about 55 VDC.
The two filter caps are in series, with 55 V across each one, or a total of 110 V across the pair. This is fine.
The amplifier is rated at 125 W (per channel) into 8&Omega; and 175 W into 4&Omega;. This implies that the output voltage is at most 31.6 VRMS and the maximum current is 6.61 ARMS.
That means that the power supply must be able to deliver at least &plusmn;50 V (peak) to the output stage, at an average current of 13.2 A, 18.7 A peak.


If you're seeing the DC bus voltage drop from 110 V to 85-90 V, then yes, the power supply may be a bit undersized. But if you beef it up, the output devices may not actually be able to handle the extra power, and they'll overheat and fail. Keep an eye on the heatsink temperature!