Does anyone out there have any tips on getting a better tone from the H K Puretone Combo? I'm trying to get a richer singing tone with more top end sparkle. I'm trying to get more of a Matchless/Vox tone. Has anyone experimented with different tubes in this amp? Because its a Class A amp, can I change the output tubes without having them biased.....as long as they are matched???? Has anyone tried NOS el34s with this amp?