Caution: This is a long post :)
Here are some comments concerning amplification that I would add to this excellent discussion, which also have potential implications regarding speaker selection:
First, small box-type two-way speakers (such as those the OP is particularly interested in) are very often designed such that their impedance in the bass and mid-bass regions is significantly lower than their impedance at higher frequencies. The intent being that when such a speaker is driven with a solid state amp the near zero output impedance of nearly all solid state amps will result in greater amounts of power being delivered at low frequencies than at high frequencies (for a given input voltage to the amp), thereby augmenting what would otherwise be a relatively weak response at low frequencies.
Since the output impedances of tube amps vary widely, what that means is that if a tube amp is to be selected for use with such a speaker it should have relatively low output impedance (for a tube amp). And since amplifier output impedance is inversely proportional to damping factor, damping factor should be relatively high (for a tube amp). Otherwise weak bass and over-emphasis of higher frequencies would be the likely result with many and perhaps most such speakers.
If a tube amp is chosen for use with such a speaker I would suggest a damping factor of at least 8, and preferably more. Most or all of the Quicksilver amps that have been discussed meet that criterion. Other tube amps having relatively low output impedances/high damping factors include those made by Audio Research, Music Reference, and McIntosh.
Second, I’ll mention that a bit more than a year ago I purchased a Pass XA25 from Mark at Reno HiFi, and I could not be more delighted with the amp as well as with the purchase experience. It replaced a somewhat older but very highly regarded tube amp, a VAC Renaissance 70/70 MkIII, which I had used for the previous seven years. (That amp cost more than $14K when it was new, ca. 2000). With my particular speakers (Daedalus Ulysses), at least, it comes remarkably close to the VAC with respect to dimensionality, imaging, liquidity, and other traditional fortes of high quality tube amps. It’s a bit less rich sounding than the VAC, but I interpret that as an increase in accuracy, which is fine as far as I am concerned. A caveat, however, is that my speakers are rated at 97.5 db and have an extremely flat and benign impedance curve.
The INT-25 integrated amp of course combines a very similar power amp with a line-stage preamp.
Also, per Stereophile’s measurements the maximum power capabilities of the XA25, the INT-25, and the INT-60 are all within about 1.2 db of each other, into both 4 and 8 ohms. I suspect Mark at Reno will tell you that the XA25 and INT-25 sound better when operating within their “class A envelope” than when asked to provide an amount of power which causes them to transition to AB. The class A envelope for those amps is spec’d as 50 watts peak into 2, 4, or 8 ohms. (That number would be significantly less on the more usual rms basis). However the INT-60 is described as being primarily a class AB amplifier anyway, which leaves class A at 30 watts. (It isn’t clear if that is peak or rms). So all three amps seem roughly comparable in those respects.
Finally, per the following calculator which I referenced in one of your other threads …
https://myhometheater.homestead.com/splcalculator.html… Two speakers having 85 db/1 watt/1 meter efficiency, if driven with 25 watts and with a bit of “room gain” factored in, can produce an SPL of 95.3 db at a 10 foot listening distance. 130 watts (Stereophile’s measurement of the XA25’s capability into 4 ohms, which is a bit higher than that of the INT-25 and INT-60) would produce 102.5 db. Both those numbers are probably sufficient for most users on most or all of their recordings, but will be marginal **at best** for some listeners on some of their recordings, particularly those having very wide dynamic range (such as some classical symphonic recordings which have been engineered with minimal or no dynamic compression).
All of which, IMO, reinforces the notion of considering higher efficiency speakers. Especially given that the OP already has a sub, which would help to compensate for the tradeoff that often exists between the efficiency and the deep bass extension of a speaker, for speakers that are of similar physical size.
Best regards,
--Al
