As for matching a preamp to an amp, I've always looked at impedance specs, but lately, I have also come to the conclusion that sensitivity and the gain in the amp are important as well. Typically, I have heard that if the input impedance of the amp is greater than a 10:1 ratio to the output impedance of the preamp you're okay. However, while my components have that ratio covered, the input sensitivity of my amp is .8V and the gain is 30db (which really suits it more to a passive than an active linestage in my opinion). The result is that even though my preamp gain is around 10db, I can't get to far around the volume dial before the music gets too loud. There is little flexibility in the adjustment as well.
Looking at the specs on your amp input impedance is 137k ohms and sensitivity 1.7V. In this case I might try a tubed active linstage with 10 - 15db of gain (adjustable if possible) and 600 - 1200 ohms output impedance. Come to think of it these specs match up nicely to something like a Joule Electra LA-100 MkIII which is what I use. Although I am evaluating passive preamps at this time, I am not certain this would be a benefit to you, especially if you can go direct from your source. The advantage of a properly designed linestage would be increased dynamics by virtue of that linestages sonic signature. The Joule for example is very musical, and while more transparent than other tube preamps, will impart a sonic signature. The additional cabling will have an impact as well.
I too have gone direct to the amp, but using a pair of EVS Nude attenuators in between, which are hooked up via the amps inputs. This is something you may want to experiment with as well as it is a relatively cheap option if all you have is a single source. EVS are no longer available new, but if you can find a set used it's worth a try. Endler attenuators are available new for $160 with a 30 day trial. I have used both and each has been very good. If you like transparency this is a great option.