Yeah, I tried Solen MKP just to see how much worse a cheap cap would be and it's way worse than I thought it would be. It did a lot of damage to the sound.
BobM, the Mundorf S/O was pretty good in my amp but the RelCap PCU is quite a bit better, and 2x the price as well.
For less expensive (Mundorf s/o range) options Feastrex was offering a cap... Urushi I think... looked like a PP cap with some damping added. It was very, very good. I won a pair at rmaf 2 years ago and tried them out.