Amp/pre compatability

It is the pre-amps output impedance that is important relative to the amps input impedance. Considering you amps 50K input impedance, if the pre-amps output impedance is less than 1000 it should be OK. Less is good. If it is 3500 or more it could be a problem. Could be, but it might be OK.

Matching the preamp output impedence with amp input impedence. The preamp's input impedence (50K) means nothing here. So your preamp's output impedence is ???. You amp's input impedence is 15K (relatively low). Many use the 10X rule, which would say that you are find as long as the preamp's output impedence is lower than 1500 ohms. I don't agree with that rule though. Generally more is better and I've heard rolloff at 20X's. You should be alright if your preamp output impedence is under 600 ohms.