You should be able to do it (with less than optimum results IMHO) by getting a ss amp with more input sensitivity than the tubed amp and then putting an attenuator between the preamp and the ss amp. Using this attenuator you could come close to matching their output. You would then use the attenuator on the preamp to control both amps. BTW, if your preamp has two outputs be sure you know how they are configured. It ain't all that straight forward (I found out the hard way). If you can't find out, then take the signal out of the preamp using a 'Y' connector.
Hope that helps a bit. Perhaps others will have other ideas.
Hope that helps a bit. Perhaps others will have other ideas.