NEED HELP! How to circumvent Onkyo M-501 Single Channel????

[80'sAudio]

Hello to all, hope you can help with an interesting problem I have. To some it may be a no brainer, in fact I hope it is.

I have an Onkyo M-501 Amp. It's manual states I cannot use both A & B channel without using the Onkyo P-301 Preamp. The 301 preamp sends a signal into the 501 amp's remote input via a connecting wire. The function on the 301 is called a Multiple Room Remote. I guess they never figured someone might want to use BOTH channels in the same room, to maybe power a twin channel sub?

I can't imagine I'm the only person to have ever asked this question and I know the answer is out there somewhere. I would greatly appreciate an answer or a redirect to where I can find out. Thanks in advance, Lee