Im trying to plug my amp head into a combo speaker (disabling the combo head obviously) but it isnt working. any suggestions?
It isn't working... oh my. I hope you matched the ohms resistance, or (if you have a tube amp) you may well have just cooked your output transformer. If it is matched, make sure you are using a speaker cable. If the speaker is blown, I doubt you would be getting do sound and there would be visible damage.
OMFG this is fail of the year..

1 i connected the 6505 to the marshall combo head ..i thought because it said speaker input it would bypass the marshall because it was disconnected.

2 i was blasting the 2 amps at 16ohms on the 6505 and 4 on the marshall...and the sound was coming through my hifi..which wasnt plugged into any of the amps!!

however it seems to be okay and the marshall speaker is much nicer. anyways im selling both of them.
I am not quite sure what to say to that. Better sell them both as damaged goods...
Alright, what you did was a huge no-no. The Peavey was not under load. This will destroy the output transformer. Then, by plugging it into the Marshall, it likely made the impedance mismatch from hell, damaging the Marshall. As for the sound coming out your hi-fi, I have no clue...