Yes, but varies based on the quality of the adaptor. I would recommend WBT or Cardas.. or have a professional install an extra output on your preamp.. not a big job at all.
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Are you using the same set of speakers? I would worry about the other end. If you are connecting two sets of amp outputs to the same set of speaker terminals, that strikes me as a bad idea, because the amp that is on will be putting power into the output of the amp that is off. Seems like the potential for distortion at a minimum, and, at worst, the kind of thing that might blow up an amp...
It is really a question of impedance.
In theory a preamp has a low output impedance and likes to see a high input impedance (10k ohms or more).
If you use a Y adaptor you can halve the input impedance of the power amp as the preamp sees the two power amps' impedance in parallel.
Reduced input impedance on power amps can manifest as reduced high-frequency clarity and a loss of dynamics. Using interconnects with the minimum capacitance can help to mitigate this effect. Something like the Chris Venhaus DIY interconnects with eichmann bullets would probably be the minimum capacitance.
The best thing to do would be to get a relatively inexpensive Y adaptor and try it, and if it sounds OK get a more expensive one made up. I certainly would not have the preamp modified to add extra outputs, since these most likely are simply extra jacks wired to the existing output and do nothing to solve the impedance problem, but could reduce the resale value of your preamp.
I would suggest you get the Y-connectors at Radio Shack and see if they work for you. If the input impedance of the SS amp (even when off) is much lower than that of the tube amp, it will shunt most of the signal and you will get little out of the latter.
If it works, then you can consider investing in higher quality connectors.
Kr4...No Kal. The low impedance SS amp will draw more current from the preamp, but the voltage will be the same at both amp inputs. If you doubt this, make some measurements. The loading of the preamp will be the same regardleass of whether the power amps are on or off.
But Radio Shack Y-connectors do work fine.