Most modern High-end Opamps like the OPA604 have a working voltage of + and - 15 Volt instead of + and - 12 Volt. The power-supply for the Opamp should be increased from 12 to 15 Volt by changing the Zener-diodes D1 and D2 with 16 Volt zeners. Otherwise there will be clipping at higher input-voltages and the undistorted power will be limited to about 70 Watt per channel.

Modern Opamps eat more power. This may cause unwanted switch-off behavior ("Plops" in the speaker) when switching off the amp. See the other posts on my blog for solving this matter.
In order to stabilize a "modern" Opamp one should add 2x 100 nF capacitors between pin 7/4 of the Opamp and the input-stage mass to avoid the input-stage oscillating and C3 has to be removed.
For the local feedback I have the best experience with Bernd Ludwig's proposition (see the links on the left): replace R6 with 100K and C4 with 150 nF.
For the general feedback I use 10 K for R4 and 47 mF bipolar for C2.
This will get you an input-sensitivity of +/- 1 Volt, which is ideal for modern pre-amps and sufficient for the Quad 33 as well.

http://www.quadmodsusa.com/Pro26Tech/405upgrades.html