Passive & Tubed pre-amps to power amp at same time?

Not a good idea. The output impedance of the preamp that is not being used will be seen as a load by the preamp that is being used, and most likely at least one and probably both of them would be unable to drive that load with good sonic results.

The output impedance of the passive preamp will vary widely depending on whether it is resistance-based or transformer-based, and on the setting of its volume control, and on the output impedance of what is driving it. But the preceding paragraph is almost certain to be applicable in any case.

Regards,
-- Al

I'm sure someone here can give you a better technical explanation, buy I wouldn't do it. With everything powered up at once, the both sources would interact with each other. You need to keep them separate. A better solution would be to get a passive that accepts 2 or more inputs, and use that to switch between the CD player and pre amp. The downside would be that the passive will be in the signal path of the preamp, and you'll have to set the volume on the passive whenever you need to use your preamp. You would have to do some experimenting to see what setting works best.

Al, Zd - THANK YOU both very much.

Al, I was concerned about a potential interaction and absolutely hadn't a clue what the nature of that interaction might be. Appreciate your explanation.

Zd - I think you have given me a good alternative. I'm thinking about getting the Schiit SYS. This does have 2 inputs. What I've read so far seems to indicate the Schiit is pretty transparent so might be OK even if it's in the tubed pre signal path all the time. BUT given what Al has said, not sure what effect if any the passive might have on the tubed pre-amp. If I got it, would have couple of weeks to figure out if it's OK given Schiit's return policy; maybe more if I got it through Amazon.

Al - I'd certainly appreciate your comments about Zd's alternative.

The Sys has a specified input impedance of only 10K, which will be too low for many tube preamps to drive with good results. For example, if like most tube preamps yours has a coupling capacitor at its output, its output impedance is likely to rise to high values at deep bass frequencies, which is likely to result in significant rolloff of the bottom octave or two with a 10K load. Also, the 10K input impedance of the Sys, as seen by the tube preamp, will actually be somewhat less than that depending on the input impedance of the power amp and on the setting of the Sys' volume control.

Perhaps the best thing to do is to get both a passive preamp not having a switch, and a separate line-level switchbox. The switchbox would select which of the two preamps has its outputs routed to the power amp, and would connect the outputs of the preamp that is not being used to nothing. A number of switchbox possibilities are suggested by me and others in this thread.

Also, regarding the passive preamp itself, and assuming that it is resistance-based (as opposed to the probably much more expensive transformer-based types), keep in mind that perceptible rolloff of the upper treble, and consequently sluggish transients, will result if the combination of cable length and capacitance per unit length of the cables at its outputs is not kept low. And if a switchbox is used between the preamps and the power amp, that would mean the total lengths and capacitances of the cables connecting the passive preamp to the switchbox AND those connecting the switchbox to the power amp.

Good luck! Regards,
-- Al

Al - That's a lot for a non-EE to digest. Thank you for the detailed additional input. Much to ponder. Might be simpler to just live with things as they are; i.e., limited usable volume control range on pre-amp with CDP (but things do sound great).