I have heard the same too, so a decent tube amp can keep the bass right and get the low freq?
Stereophile bench test:
" On the other hand, the Sasha's plot of impedance magnitude and electrical phase (fig.1) reveals the speaker to be a demanding load for the partnering amplifier to drive. Not only does the impedance drop below 4 ohms for most of the lower midrange and upper bass, with a minimum value of 2 ohms at 86Hz, but there is also an amplifier-unfriendly combination of 3 ohms and –43° phase angle at 61Hz."
Lower bass if you want it kunalraiker clean, tight, rhythmic and punchy, you'll need an amp with current, sure a tube will work and mostly sound good, but the best will not be extracted from the Sasha's in the bass.
That 3ohms with -43 degrees of phase angle will look like 2ohms to the amp that's driving them.
Cheers George