I Have A Tube Pre-Amp Which I Like To Use And Just Want To Use The AMP Section Of The Monoprice 50 Watt Stereo Hybrid Tube Amplifier with Bluetooth and Line Output.From what I can see and read in the user's manual, this amp has a built-in pre-amp. It has a direct wired input for CD and also a pre-amp output (from it's pre-amp). You could use another amp, but would need one with it's own built-in pre-amp. If this amp didn't have a pre-amp, it would most likely not have it's own volume, bass and treble controls. Those functions occur in the pre-amp stage. Also, dedicated power amps only have one input - that from the pre-amp.
What source do you want to connect? Is the reason you want a pre-amp because this one has few inputs? The CD input for example, can be used for just about anything except a turntable.
I see. Then you can take the output from your preferred pre-amp, connect it to the CD inputs on the Monoprice amp and there you go. Just adjust the volume control on the MP amp about 2/3-3/4 to max and use your preferred pre-amp to control the volume. The only reason I can imagine you'd want to do this is for more inputs. The pre-amp in the MP unit cannot be bypassed for use as a power amp only. Ideally, get 2 dedicated mono-block tube power amps or one stereo tube amp.
H Would I Input The headphone jack Into The Monoprice