Amp issues - tubes/caps or something else?

I have an 87-X modified by Friedman and it's developed a trait where it effectively sounds like someone turns the presence down from (say) 7 to 3, so things get muffled for a couple of minutes, and then it returns to normal, then the cycle repeats.

I've replaced the pre-amp tubes but is this likely to be a power tube thing? The effect isn't apparent on the clean channel if that's an indicator (but then it might not be as obvious with the lower gain I guess).

I've never had to deal with biasing before as this is my first Marshall (pretty much all mesa previously), and have not experienced the need to change caps so not sure what the effects of those failing might be. Will probably just take it to a tech to get it looked at, but I don't want to go with blind trust in what they might decide to charge me!

I'm suspecting it might be your power tubes.
Best option would be to ask a qualified amp tech.

Check the filter caps. Motorboating is a sign they may need replacing.

so it doesn't lose volume you just lose treble ? Does it do it matter where the presence is set ? First thing I would do is turn the treble and presence controls and see if anything changes . You may just have a dirty/bad pot .
If you have defective tubes pre or power It should effect the volume more so than the tone .