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

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

If the 91dB mentioned above is the sensitivity, then all that speaker needs is 1 watt to produce 91dB sound level at 1 meter. So there's no surprise that a 12 wpc amp can and will sound loud (clear is a subjective term) enough in a moderately small room. Other factors, which one could argue that are more important, are the speaker's efficiency as well as the way its impedance behaves across its frequency spectrum. Throw a piece of content with wide dynamic range and lots of transients, something like a large orchestral western classical music content, and the weaknesses become glaringly clear. Vocals, single instrumentals, and small bands like jazz trios, etc., not so much.