HK 730 right channel quit working (all inputs and outputs including headphones - preamp?)

I've had my new-to-me HK 730 powering my also-new-to-me Infinity RSIIIBs for a few weeks now, and have been extremely happy with the sound. This morning it wouldn't play out the right channel except very (VERY) quietly out of the mid and tweeter. Aux 1, FM and AM all the same; same with using Speakers 1 and 2 outputs and with headphones. I've been planning to do the phono EQ curve mod but this is a real bummer. I never really pushed the speakers too hard as I live in an apartment complex; it was working fine last night but I left the receiver on for a while (1-2 hours I think) after I was done listening to music before shutting it off for the night. I assume since the headphone output is the same as the speaker output something in the preamp got messed up. Any tips for diagnosing it would be greatly appreciated; I'm fairly handy in general but electrical stuff isn't my strong suit. Would also appreciate good repair tech recommendations in VA (preferably Roanoke/Charlottesville area) if it'll require a complete refresh.