I am TORN between Tube power amp and SS power amp.

rodulfo, the SET amp you mentioned raises three red flags; input impedance of 50K ohms, 9.2 wpc power, and the weight of ~25lbs. Normally the weight of a SET 300B is a good indication of the quality of the transformers which are the most critical (and expensive) component of the unit. And the 9.2 wpc will be just enough to tickle the woofers on the L100. 

I am running JBL L100 (Alnico, straight line speakers). In the manual, it says sensitivity is 78dB/1w @ 15 feet. I read somewhere that it is equivalent to 90-91 dB/1w/1m.

That is correct. For a box-type (non-planar) speaker it can be calculated that 78 db/1w/15 feet corresponds to 91 db/1w/1m.

However I agree with the red flags cited by Kalali, and unless you listen exclusively at low volumes and to recordings having narrow dynamic range a 9 watt SET will not be powerful enough for 91 db speakers.

I assume from the quoted statement, btw, that your speakers are original L100s, not the "L100 Classic" that apparently is being produced currently. The latter having specs of 90 db/2.83 volts/1m and a 4 ohm nominal impedance, which corresponds to only 87 db/1 watt/1m.

I couldn’t find an impedance curve for the original L100, that would show how its 8 ohm nominal impedance varies over the frequency range. But the VTA ST-120s relatively high damping factor (for a tube amp) of 16, which at least theoretically corresponds to an output impedance for an 8 ohm tap of 8/16 = 0.5 ohms, suggests that it is likely to be able to interact with the speaker’s impedance variations in a reasonable manner. And its 270K input impedance would certainly be a suitable match for your line stage.

Also, I found the listing you referred to for the DH-500 upgrade, and I see that the upgraded amplifier has a specified input impedance of only 33K, rather than the 47K of the original. I would definitely not pair a 33K input impedance with your line stage.

So while I haven’t researched comments on the sonics of the ST-120, if those are encouraging that amp would seem to me to be the clear choice among the three possibilities you cited.

Good luck. Regards,
-- Al

P.S: I would add a fourth red flag to the three that Kalali cited for the SET amp. Its 9.2 watt power rating is based on THD (total harmonic distortion) of 10%!!   So it is likely to be a significantly less powerful amp than a 9.2 watt rating would usually suggest.

Regards,
-- Al

My 91db speakers sounded fabulous and played plenty loud and clear with a Dennis Had 12 watt (or maybe 17 watts...not sure) per side amp, but I switched to 99db Klipsch Heresy IIIs mostly to keep Almarg happy, as I deign to avoid his ire. 

:-)

Best regards,
-- Al