Capacitors for HP filters in tube amps recommendations?

Personally; I never paralleled capacitors, unless they were identical, to avoid any possible time-constant disparities.

That's not a concern... capacitors in parallel will just sum in value (like resistors in series)... the source and load won't see any difference. In fact there are good reasons to parallel capacitors. It's easier to match the values for left and right channels (more combinations) plus statistically you'll get a tighter tolerance than specification (although you'll need to parallel quite a few to make much difference). You can achieve higher capacitance than is commonly available for a particular dielectric (I have used 5 x 10uF wima for DC blocking). Also multiple smaller packages will have a larger surface area than a single large package so heat dissipation is better.
If you want to go down the route of matching your own for around $25 you can get a cap meter that will measure 1pF to 20,000uF although you might want to factor in a few precision caps to use for calibration.

I said, "personally".        You’re welcome to your opinion, if you have one regarding time-constants (which is to what I referred, but- you never addressed) and anyone else’s, that you value.

I’ve read concerns from a couple of amplifier designers about time constant ramifications resulting from the use of bypass caps. They state that contrary to the opinion of some audiophiles, bypass caps are to be avoided whenever possible. The obvious question is: "What designer(s)?" I don’t at the moment recall, but it was a couple of biggies, on the level of Nelson Pass, if not in fact he himself.

I just prefer not to give myself a reason to think about possible timing/phase anomalies, anywhere in my music's path.      That's been my stance, since the early Eighties.       Like everything else; opinions vary.       

By the way, I too have bought parts from Michael Percy, the last time earlier this year. I hope he's doing okay, he's a great source.