You're on track.. you just need to find a receiver that has pre-outs (not just tape loop) for the main L/R channels. That typically means it will have pre-outs for all the channels.
You can hook your stereo sources up to either your current tube pre-amp, or the receiver. I'd keep them connected to the tube pre-amp, and just connect your HDMI sources to the receiver.
When you're ready to use multi-channel, you'll just select the Pass-Through input on your tube pre-amp, and the receiver would control the volume and input switching for multichannel sources.