Here we go again with perpetual preamp output impedance question.
The 10:1 "rule of thumb" represents reasonable guidance that's readily supported by theory. If the Stratos input ("load") impedance is 10K, the "source" impedance of the device (preamp) should not exceed 1K.
Where this gets dicey - and confusing - quickly is in the specsmanship of the preamp manufacturer or seller. Some will quote a true "source" impedance, while others will quote a recommended "load" impedance, with most not precisely stating which their number represents.
The "source" impedance is that which results in the voltage across it being half of that which would exist with the device operating into an open circuit. Practically speaking, you run a tone through the device at low level, measure the output voltage with a high-impedance voltmeter (nearly all digital multimeters - DMMS - are), and connect resistors of decreasing value from 1M downward until the voltage is reduced by 50 percent. That resistance is the "source" value. To be precise, impedance has resistive and reactive components, but reactance is negligible in preamp outputs and power amp inputs, except at the farthest frequency extremes, so it's not relevant here.
But the stated "output impedance" of the apparatus is just as likely to be the "load" value that the manufacturer considers the minimum for proper performance. If the "source" value is 1K, the recommended minimum load is usually 10K or more.
So the poor consumer is likely to be confused: if the spec sheet says 10K without any qualifying words (e.g., "source" or "load"), is the proper minimum rating of the device being fed 10K or 100K? Hard tell without knowledge of the circuit topology or the test equipment to just go measure it.
Anyway, with regard to the present discussion, some generalization is possible. If the tube preamp utilizes the "cathode follower" configuration in its final amplifying stage, it should be happy with the 10K load of the Stratos. However, if the last tube stage is a "common cathode" amplifier, it's not likely to play at its potential with the Stratos, if it uses common tubes like the 6DJ8, 12AX7, etc. There are tubes with low "plate resistances" that are quite comfortable with a 10K load, but they're uncommon in audio applications.
I could continue to write on the theoretical nuances of component matching, but this is probably too far into geek-talk as it is. As Klaus so often notes, synergy is everything. Some of that is achieved by ensuring that components are matched to each other from a technical perspective. The rest is, well, an art, maddeningly frustrating, fun adventure, or a little of both.
Karl
