@charles1dad Those are good questions! For a cost no object, I would likely go the exclusive film cap route in the power supply- since even the best 'lytics will tend to drift with age (however good quality modern caps last MUCH longer than those of the 70's and 80's). What I like to do in most cases, is use electrolytics bypassed with small film in the first stages where massive filtering is required, and then have the last stage be pure film since it's the last part of the filter before the audio stages.
As far as coupling? That's another tough one. Inter-stage trafos are technically superior in the phase region (where multiple stages are required), and also when you need a step up or impedance matching transformer. However iron has it's drawbacks in terms of frequency extreme rolloffs, so you trade "possible" phase issues for "possible" response issues. However in practice using good quality parts- these issues are small and splitting hairs so long as the design is done right.
I for one think that high quality caps can get a design that sounds phenomenal so I typically don't look into the interstage transformers much. However, on a tube that requires a huge voltage swing and lots of low current to drive it (ex: 845, 805, GM70), it's almost necessary to use an interstage trafo as there's so much in the signal path otherwise.
As far as coupling? That's another tough one. Inter-stage trafos are technically superior in the phase region (where multiple stages are required), and also when you need a step up or impedance matching transformer. However iron has it's drawbacks in terms of frequency extreme rolloffs, so you trade "possible" phase issues for "possible" response issues. However in practice using good quality parts- these issues are small and splitting hairs so long as the design is done right.
I for one think that high quality caps can get a design that sounds phenomenal so I typically don't look into the interstage transformers much. However, on a tube that requires a huge voltage swing and lots of low current to drive it (ex: 845, 805, GM70), it's almost necessary to use an interstage trafo as there's so much in the signal path otherwise.