I've used the method suggested by Rwwear and it works great. Given it costs you nothing, it should be the first thing you try.
Assuming your tube amp doesn't have HT passthru, you simply connect the Front channel outputs from your HT pre/pro to one of the line level inputs on your 2 channel preamp. Set the volume control on the 2 channel at 12 o'clock, or some other easy to replicate position, and calibrate the output levels of your pre/pro at this setting. Then when you want to listen to HT, simeply power up your pre/pro, select the pre/pro input on your 2ch pre, and set the volume level.
I've since purchased a tube preamp with HT passthru - which is nicer, but I suspect you'll find the above method very satisfactory.
Assuming your tube amp doesn't have HT passthru, you simply connect the Front channel outputs from your HT pre/pro to one of the line level inputs on your 2 channel preamp. Set the volume control on the 2 channel at 12 o'clock, or some other easy to replicate position, and calibrate the output levels of your pre/pro at this setting. Then when you want to listen to HT, simeply power up your pre/pro, select the pre/pro input on your 2ch pre, and set the volume level.
I've since purchased a tube preamp with HT passthru - which is nicer, but I suspect you'll find the above method very satisfactory.