PREAMP technology shift? - Class A/ balanced/ dual-mono/ zero feedback vs. op-amp preamps

Actually that rule of thumb is 10:1. So if your output impedance is 500 ohms you should be able to drive 5000 ohms.

There is a complication with many tube preamps, which is the size of the coupling capacitor at the output. The issue is that to drive 10K ohms (which most of the time is the minimum you will see in a consumer amplifier) you need a pretty big coupling cap at the output of the preamp if you want it to play bass (which, if the cutoff is 20Hz, will be 10uf; but to avoid phase shift in the audio passband 100uf is recommended. You're not going to find very many preamps with a value that large- they usually split the difference in some way with 20uf being about as big as you're going to see). The bigger the cap, the more its going to color the sound so every manufacturer has a different hedge on this. For example, ARC noticed that many solid state amps have a higher input impedance and so limited their preamps to about 30K as a minimum in order to play bass right. Tube amps are generally 100K and some tube preamps have commensurately smaller coupling caps as a result, ignoring the solid state market altogether.

If there are tests available that plot the output impedance, you can see this by looking at the 20Hz output impedance as opposed to 1000Hz which is usually the frequency at which this spec is usually stated. The frequency plot will tell the story- often the 20Hz value will be multiples of the 1000Hz value. If its still 1/10th of the input impedance of the amp you're OK.

We avoided this by developing a direct-coupled output circuit for which we got a patent. As a result the output impedance vs frequency curve is a flat line- the same at 10 or 20Hz as it is at 1000Hz or 10KHz.

Just wanted to mention a few of you have given misinfo on the schiit freya, it doesn’t only have a Jfet solid state output stage, it actually has 3 seperate output stages you can change between. Fully passive, jfet buffered 0 gain, and tube gain output

Specifications may or may not (most often not) relate to great sound quality.  2 different preamplifiers that measure similarly will often sound very different. If you limit your search to "big name brands" you may be missing some opportunities.  To say nothing of the fact that as your original question regarding "design philosophies & circuit topology" render a cloudy picture. As a sidebar the audio industry has no standards on anything.  Even preamplifier output values.  IMO preamplifiers are the weakest link in the audio chain.  I have always been a proponent of full-featured preamplifiers and single ended vs balanced.  Unless you have no aspiration on spinning vinyl then you can simplify your search to line level devices.  Concentrate on in home auditions and listen. 
Good luck!

I would just cut to the chase and go Pass Labs.  The integrity and dedication of this man over the years,  and his willingness to share with the DIY community,   and the consistent quality of his product makes it almost a no brainer IMO.    I have a class A amp of his and everything about it is  just simply art in every respect-- which of course its sound reproduction is paramount.     I have cut back on my "habit"  as I am working on making a major move out of the area otherwise I am itching for a Pass preamp. 

^ I agree on Pass.  My first the Threshold Stasis 3, then the Aleph 3 and 5, then the great X150 (probably the best used 'cheapish' general amp one could locate).

The P and L and Xono and others have been great preamps, LS's, and phono stages as well.  I've 'slummed' for 20+ years with the Pass designed GFP-750, which died (accidental).

Retired now, so the budget has taken a hit, so the Freya looks like a good choice to go w/ my X-150, Schiit Gumby. Been using the Ragnarok as a line stage, so the Freya will be a step up.

In terms of Schiit, like to see them make an amp that runs class A up to 25 watts, and A/B up to 100+. The Vidar (in mono) and Ragnarok do not handle loads under 4 ohms.

In terms of tubes.  I like them in pre-amps (ARC SP-15 onward) but never could stomach tube amps - even very expensive ones in the bass, and retubing cost?  My lord...