However, I have since been told that your amplifiers only perform optimally when linked with one of the preamps of your own design, unfortunately.I have no personal experience yet with Ralph's products, but fwiw I believe I can state with certainty that there is no technical reason why that should be so. Both your Meridian and Ralph's preamps are fully balanced, and have low output impedance. The amp will not know the difference, although of course synergy between the sonic characters of those and other components in the system will come into play.
Given that you want a tube amp, I would expect that apart from the intrinsic sonic character of the amp itself, by far the most important interaction that is involved will not be between amp and preamp, but between the amp's output impedance and the impedance vs. frequency characteristic of the speakers. I could not find an impedance curve for the 17.5's, but if you or someone else could locate and provide a link to one I think it would definitely help to focus the recommendations you will receive.
Regards,
-- Al