I would suggest starting with the 0.1uF/600+v interstage coupling capacitor. Being small, it is less expensive to play with You may want to try a few different types - oil, plastic, mica, etc. Don't hesitate to lok at inexpensive non-audiophile-approved caps, such as mylar (polyester) - you will at least learn why they are not popular.
Second set of experiments - different values of parafeed capacitor. Stock is 1.5uF/600+v, but the range of 0.8uF to 2.7uF is worth exploring if you have speakers. There is some interaction in the deep bass between speakers and parafeed capacitor value; it may or may not be present with your speakers. I'd do this first with Solens, same as stock, just to find the value that gives you the most satisfying bass. Once you know that value, then in makes sense to try a different type. For some unknown reason, the most satisfying parafeed caps are often different from the most satisfying interstage caps.
Don't forget, most caps will take at least 50 hours of actual music before they are broken in enough to judge them - they almost all sound thin and nasty at first. Some types, like teflon, take ten times as long. It's a long, slow process!
