Hi Steve,
Paul is on the money; any tube pre with a cathode follower output stage will find amps with Zin down to 10K a walk in the park. The Soraya Zin is around 43K, pretty high, so easy to drive. Thanks Paul!
The tube preamps that struggle are capacitively coupled from a plate loaded triode; most of these don't go below 2K Zout, and these really need around a 20K load on the amp to cope. Of course, this depends on the tube, but even if you used a big 300B, the Zout would still be around 500R, pretty high.
The best tube circuits in terms of drive are transformer coupled common cathode. Let's choose a tube of plate impedance 5K, about average, and mu of say 40. Let's put a transformer into the plate circuit, and run it 10:1 step down. This would give us a gain overall around 4, that is 12dB, which is about right for a line level tube preamp. The Zout of a transformer depends on the primary circuit divided by the square of winding ratio, so if the Zout of the tube itself were 5K, then dividing this by 100 gives a Zout around 50R.
You would load up the secondary with a 150R resistor (this reflects to the plate at 15K, about right for the tube ratings), and this transformer coupled plate loaded triode would give a very good performance into almost any load - and no caps!! Of course, the trafo would need to be good quality, gapped, and well made in sections for best performance.
Cheers,
Hugh
