Amp preamp impedance matching...can anyone explain?

As a simple rule of thumb most any transistor preamp can drive any tube amp.

Its not always the other way 'round as pointed out above. (FWIW we get around the problem by direct-coupling and using paralleled tube sections to obtain a low output impedance that can drive any amp around.)

But if there are coupling caps its a different ballgame. For example, ARC recommends no less than 30K as the input impedance of the amplifier with which the Ref 5 is used.

10:1 is a safe minimum value, but you can go 100:1 and its fine. So in the ARC example above, standard engineering practice suggests that the output impedance of the Ref 5 is 3K ohms. However they are fairly conservative and are showing 600 ohms on their website, although that might be at 1KHz. IMO the 20Hz output impedance tells the real story.

The lower the output impedance, the more the line section can control the interconnect- by that I mean the less sonic attributes the cable will have. So the lower you can get the output impedance, the better (all other things being equal...).

Al, isn't it the case that ARC does not recommend less than a 30K load with their preamps? I know that is true of at least one of the recent Ref series.

Bifwynne, that is a common problem if you are trying to use a balanced preamp with a subwoofer. Many subs only have SE inputs.

Another solution is that Jensen transformers makes a transformer that is optimized for subwoofers. It has a fairly high impedance input that can be run balanced or SE, and has bandwidth to less than 2 Hz so it does not mess up the bass.

But I think you are lucky you ran into Tom Tutay. He does good work :)