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.

Transformer coupled output is an excellent approach, especially for tube preamps which otherwise contend with the noted issues of output caps and rising impedance. The Ren V claims an even 150 ohms or less over the entire music bandwidth, and so should have no problem driving any SS or tube amp. Using transformer coupling for both inputs and outputs also frees up the preamp circuit to use “only” 2 tubes, and handles SE to balanced (and the reverse) conversions “for free”. 

@dpop it specifies a recommended load impedance of greater than 300 ohms. That includes loads like 10K, 50K, 500K, etc. This is not like matching taps on power amp transformers, where maximum power transfer is the goal. Here we are only concerned with transferring voltage signal, where Ohm’s law dictates a good transfer. That means you want as low an impedance on the source side (150 ohms being very low for a preamp, especially at 20 Hz), and a “sufficiently high” impedance on the amp side (300 ohms being far lower than typical). It is reasonable to aim for a load impedance 20x (or more) greater than the source impedance to minimize losses caused by voltage division.

It’s absolutely not needed here. In fact, I think it would be a very bad idea to feed a Ren V into 300 ohms - that’s the lowest impedance that it will work “ok” with. I’ve used the Ren V into 100K ohm tube amps no problem. I now use the VAC Master, same circuit, same specs, same deal (just better sound quality from premium parts and PSU). Also works great into a SS amp. The “> 300 ohms” spec is intended as a flexibility boast, not a requirement or limitation. Many tube preamps will require a minimum load of 10K or higher, which can become a restriction with some SS amps.

@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.

@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.