LSS hissing noise

[Shooter]

I have an LSS, purchased used, with very little use. It had sounded great until recently. A faint hiss started while on channel 1 only. I did a little research and decided it was a tube problem. When ordering a replacement tube I decided to just go ahead and change them all.
Tubes came in tonight, all except the 5Y3 which is on backorder. I replaced the 12AX7's and EL84's, but the hiss is still there. I dug into it a little deeper and found out that if I switch the effects loop out of hard bypass the hiss goes away. I have spent the last few hours searching, but have not found any info on this problem. I plan on calling them tomorrow, but any help would be appreciated.

 

[rarebitusa]

Is the hiss effected at all by the selection of chan power ie: 100/50/10 (if it has 10W)?

Perhaps a long shot... as a quick test pull the 5Y3 Rect Tube, select "Diodes" on the rear panel. Power up and check if anything changes WRT to the hiss.

The other thing you can try is to isolate the preamp and monitor the fx send with an external amp to see if that is where the noise is coming from. Keep a speaker load on the power section. If that is quiet then plug a guitar into the fx return and monitor the output.