Welcome to the Fender Rumble Club, Owens W!

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Those original Rumble 100 1x15 combos are tough! The very easiest way to connect your Hartke and Rumble is to go from the effects send of your Hartke and run it into the effects return of your Rumble. This bypasses the Rumble's front end and inputs the tone from your Hartke's pre-amp directly into your Rumble's power section. Yes the Rumble will have to be turned on. Alternatively, you can go from the effects of your Hartke into your Rumble's instrument input, and go from there.
To have your Hartke power your Rumble's 15" woofer will require some permanent modifications to disconnect the the woofer from the Rumble 100 amps section, and drill and mount a jack and wire it to the woof. The choice is yours.
Rumble On!