Help on tube question

The 6550 and KT-88 are nearly the same tube. They have similar bias points and the like, the big difference being that KT-88s are supposed to be more linear ('KT' refers to 'kinkless tetrode' IOW there is no kink in the linearity curve like a 6550 has).

So you can kinda sorta mix them, but the 6550 should be matched to one of the KT-88s and that seems unlikely. If you have individual bias controls that helps- then the only issue will be that distortion will be higher in the channel with the 6550. You may not hear it though...

Don't mix them. Bad circuit leads to more trouble.

The Classic 60 was designed for four matched pairs of 6550s. It's no surprise that they worked better in the amp than KT88s.

Replace the 4 KT88's. Be safe and do what VAC recommends.

The Sigma 160i is designed with KT88s in mind(specifically),
and offers no tube type selection switch(unlike the PA160).
You may not be able to properly bias a quad of 6550s.
I would strongly recommend you not try to replace just one
tube with a 6550 either. If you do try the 6550s, watch the
plates, while trying to bias, and if they glow red, STOP!
imo: Best be patient.

I had an Audio Research Classic 60 with KT88's kept failing now and then. I replaced them all with 6550's and never had another tube failure as long as I owned the amp which was about 20 years. Could just be my bad luck but I've never been comfortable with KT88's ever since.

But yeah the tubes need to match. Don't mix a 6550 with the KT88's. Just replace them all I think.