Tak1, the concern regarding the rise that I cited in the SLP-05’s output impedance to 3400 ohms at 20 Hz (per Stereophile’s measurements), from considerably lower values at higher frequencies, has nothing to do with volume. That kind of impedance rise at low frequencies, btw, is not at all uncommon among tube preamps, and occurs as a result of the output coupling capacitor which the majority of tube preamps use. (The impedance of a capacitor rises as frequency decreases). The concern it raises is that in the deep bass region, especially in the bottom octave, some degree of rolloff and/or undesirable phase shifts may occur, if the load impedance is too low.
A rule of thumb that will assure that no such effects occur is to apply a factor of 10 to the 20 Hz output impedance, which in this case would mean a load impedance of 34K or more. That does not mean that a problem would necessarily occur if that criterion is not met (it depends on how the output impedance **varies** as a function of frequency); it means that a problem won’t occur if that criterion is met.
Also, the audibility of any rolloff or phase shifts that might occur will vary among different systems depending on the deep bass extension of the speakers that are being used. And also depending on whether or not a subwoofer is being used.
As I said in my post that John (Jmcgrogan2) referred to, "30K would be somewhat marginal (or worse) for use with several of the preamps that have been suggested." Personally, I prefer to avoid component choices that might be marginal in some way, when good choices are available which don’t have that concern. Also, John makes a good point about the fact that the benefits of the SLP-05’s balanced architecture would not be fully taken advantage of if it is used with an amp providing only unbalanced inputs and having an unbalanced internal signal path.
Regards,
-- Al
