Amp preamp impedance matching...can anyone explain?

I can’t, because I don’t know what gear that is or its intended use (you mentioned radio broadcast). A 600 ohm load requirement doesn’t sound like anything related to high end home audio, honestly. I can speak to the VAC Renaissance V because I owned one and know its intended application. It is designed to work well with as wide a range of 2ch SS and tube power amps as possible.

Back in the days of tube audio gear in radio stations, almost everything was 600 ohm transformer coupled (inputs and outputs). Some professional broadcast solid state gear was even transformer coupled (not sure why, except for maybe RFI rejection). As you can see in my link above (600 ohm output transformer termination), there is still sometimes on-going discussion on the subject, as some highly sought after vintage pro processing gear has 600 ohm transformer outputs. AKG even made a K240 headphone that was rated at 600 ohms. I’ll have to dig back into some of my old broadcast magazines to refresh my memory on the subject, since it rarely concerns me anymore.

@dpop 

It is an interesting topic - but it seems likely that studio gear with 600 ohm loading requirements (apparently to damp ringing?) is operating with very different needs (power, bandwidth?) than a high-end 2ch home audio preamp. Here we need only efficient voltage transfer; no power. Ringing apparently not an issue with good line-level output transformers into any power amp load. Again, the VAC Renaissance V is not prescribing a specific load impedance. 

I do remember that a few headphone models present 600 ohm loads, usually from Beyerdynamic or AKG, which are brands that seem a bit more concerned with studio use than other headphone makers. Most dedicated headphone amps (which are typically OTL and "high current" designs) have very low output impedance, and have no problem driving those or any other headphones. I haven't seen a 600 ohm load specified or required for any other piece of home audio gear in many, many years.

@mulveling 

You might find this article interesting:

THE 600 OHM AUDIO STANDARD, WHERE DID IT GO?

@dpop 

Right near the start of that article: "600 Ohms in order to achieve maximum power transfer to the receiving equipment". Power transfer is not a requirement in this application. It's an interesting article and I'll read it, but I also think this is all confusing the main topic here, which concerns impedance matching a preamp to power amp for efficient voltage transfer, not power transfer - all meaningful power is supplied by the power amp reacting to the input voltage.