I have an L5 amp.  What I've found is that the master volume needs to be turned up to at least the 10 o'clock position before the compressor will engage.  See if that works for you.  Of course one problem if you do this is that, at 100W, it's a loud amp!

Here is good review of the Gamma amps from Phil McKnight:

https://www.youtube.com/watch?v=zEXJeIlewNg

All solid state *analog* is now a selling point!

The amp seems OK though they should have added a reverb (which probably wouldn't have increased the cost too much).

If you want to increase the gain, add a 10 uF cap in parallel to the Jfet source resistor.  If you do this, then your diode clippers may work better.