Recently when I play my H&K switchblade, it will start to make a strange deep hum, along with what sounds almost like paper crunching lightly. Also when this kicks in, the sound will lose its presence and some volume, then kick back to normal soon after. Is this a power tube problem?
The paper crunching lightly thing sounds to me like a cable problem. I sometimes get that kinda sound when I stand on a cable or if it even just starts moving. Have you tried switching cables around?
Gonna Leave this town

Gotta leave this town

Gonna make a whole lotta money

Gonna be big yeah...
(Jimi Hendrix - Hear My Train A Comin')

I guess you've tried it with different guitars to make sure it's an amp problem first?
Sounds like it's very likely to be the power tubes, but due to the complex nature of the Switchblade there's plenty of other things that could go wrong too! I think a new set of EL34s is the way to go - how long have you been using the amp?
Could be something as simple as a loose out put jack, or a broken cable.
Quote by Altered_Carbon
That's some bony hipster sex, which may be the best kind.
Well i unplugged my guitar from the amp and it still makes the sound through the amp, so it may be the cable from the head to the cab, but its a fairly new monster cable so thats unlikely. The amp is a used amp and I have had it for about 5 months and havent really had any other problems with it. I tap one of the power tubes and it seems to take the sound away for awhile so im almost positive a new set with solve it, but these tubes are under 3 months old.