Blown Fuse After Loud Jam Session

I took the effects send (-10db mode) out of my DSL to daisy into the front end of the Epiphone Galaxie 10 (just to burn it in and loosen up the speaker) and after a half hour the fuse went in the Epiphone (no smell of anything burning) I've done this before several times but never blew a fuse. I have read some of the other fuse posts. So - I picked up (exact replacment slow blows) some new fuses and tried different power and preamp tubes and the same thing. The amp powers ups (in standby mode) and both tubes light up - but about 5 seconds after coming out of standby the fuse blows. Opened up the amp - all connections are fine - nothing visually burnt. Always a proper speaker load. I'm the original owner - amp has never been neglected (left on - no speaker load ect..) This is an inexpensive amp - however running into a greenback with a rat or TS9 goes to tape so well in the right context (I'd never gig with this amp) What do you guys think? The warranty is void because of the speaker out mod I did to it. Screen grid resistor? Does it have a solid state rectifier (it must I would think) Output tranny? Power tranny (I doubt)? I'm am lost - this this river must go to Aintree.

 

Thanks for the info guys - I just did the "no tubes test" - powered it up - took it out of standby - it made a slight unusual hum and blew another fuse. (This amp has one 12AX7 and one 6L6 if that helps anyone)