I will chime in here very late to the OP.
While I have enjoyed the interesting discourse about the high end capacitors, I think you would be very surprised to find out what is typically in the Fisher tube instrument that is feeding the whole thing.
I've restored more Fisher tube instruments over the years than I can remember at this point. These instruments are full of inexpensive, dried out electrolytics. And the coupling caps are not hand-rolled fancies or anything other than very inexpensive General Instrument polyester films. The few imported ero-fol coupling caps sometimes found in the output stage are silimarly polyesters, and prone to leakage, creating a bias collapse and serious output damage.
I've used plenty of capacitors in Fisher rebuilds over the years (including some very expensive ones), and the one that sounds the best in the post '62s is the CDE DME, a 50 cent metallized polyester from Mouser, and Sprague Atoms for electrolytics. A far cry from the mega-buck Duelunds. But much more in line with the original design. More importantly, they sound RIGHT. And when a Fisher sounds right, it sounds as good as anything ever made.
That is because Fisher voiced its designs to the parts and technology of the day. If you do this long enough, you will find that tonal voicing is the touchstone. The best designers voiced around the limitations of the available components and technologies. Sometimes, a cheap part is the best part.
People playing Dr. Stereo with their Fishers, and loading it up with designer passives often sterilize the timbre and tone out of them, and it is like staring at a glacier. The hunt for more detail and resolution can be the devil's lure.
Nevertheless, the FAR far bigger determinant to the Fisher's sound is not the passives, but the tubing. People throwing hundreds of dollars in passives at a vintage Fisher (or elsewhere) and then tubing it with JJs or Russians or other new production completely miss the point.
I do not know what you are doing to your Fishers. But it would be shortsighted to dump many hundreds into these speaker capacitors and not invest in a quad of Westinghouse 7591As in a B or C series amp section, and make the effort to roll either Amperexes or TFKs in the pre stages to fine-tine the timbre. The difference between a Valvo, Holland Amperex, Sylvania black plate, and an EH can make a hugh difference. That difference, you may find, will be far more profound than the x-over capacitor changes, and at a fraction of the cost. And you will notice that is where Avery did freely spend his OEM money. TFKs weren't cheap in 1963 either.
I recommend a recap of the Fisher, but with modest passives consistent with the original design theme. And there are things that must be done with the power supply at this point, if only from a safety perspective.
For more details, check over at AA in the vintage asylum archives, where you will find a wealth of information on the older Fishers and how to recondition them right.
One last tip. If you're a midrange fan like me, then you will prefer the B series topology over that in C. The C amp topology can be a little over extended at times. The B series and pre '63 amps and integrateds do the middle a little better. Of course, running horns, you are probably best off with a 6BQ6 model, which were the best sounding Fishers of all.
