1959 sounds harsh after blowing the HT fuse

[Mrk200]

Last winter I built a 1959 Plexi clone using the Valvestorm kit, and it sounded great until yesterday. I just acquired a Two Notes Torpedo Studio, and accidentally left the impedance switch on the Two Notes at 4 ohms while the amp was set at 8 ohm. About 10 minutes into messing around, the HT fuse blew.

I replaced the fuse, and the amp comes on fine, but there is a high frequency harshness to it now. It's particularly noticeable when striking chords, using a higher output (bridge) pickups. It's subtle when using a single coil neck pickup.

I've heard this exact sound before on this amp, and that time it was a bad bias capacitor. So I replaced both of them, but it didn't help. I've tried swapping all tubes, and all the voltages I checked were fine, including the bias voltage. My best guess is that I've damaged the OT.

Before I purchase a oscilloscope and signal generator to track down the cause, is there anything else worth checking?