I can only base my opinion on what I've played through, and that has to be the Quad.
I've tried a Mark IV and a Triaxis through a 2:90,
My old instructor's Mark I and IIC+ combos through a Marshall 1960B,
Studio pre with a 50/50, and my friend's Quad through a Stragy 400.
The only Boogie's I've owned are the ones in my sig, and I'm looking for a 50/50 to complete the rig.
Btw, I haven't fully explored the Quad's clean channels, but the .50 Cal has a sweet clean when the master is cranked & the volume is down, "Class A" or not.