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

I took a look at the manual for the Hagerman Clarinet line stage. It has a very high specified output impedance of 3K. That is most likely the output impedance at a mid-range frequency such as 1 kHz, and I see in the schematic that the design utilizes a 1 uF coupling capacitor at its output. A 1 uF capacitor has an impedance of about 8K at 20 Hz, which will result in an overall output impedance at 20 Hz that approaches 11,000 ohms!

That will make the Clarinet unsuitable for use with many and perhaps the majority of solid state amps, as well as with more than a few tube amps. I see that the stock DH500 has an input impedance of 47K, which would result in deep bass response with the Clarinet that is marginal at best (assuming that your speakers have reasonably good deep bass extension themselves; otherwise you might not notice the issue). If the upgrade you referred to would result in a reduction of that input impedance, I wouldn’t even consider it.

I know nothing about the VTA ST-120, but before deciding on it I would certainly inquire as to its input impedance. IMO 100K or more would be optimal, and as I said 47K is marginal at best.

Finally, you didn’t mention what speakers you will be driving. Its characteristics (including impedance, impedance variation over the frequency range, and sensitivity) will of course be important considerations in choosing an amp.

Good luck. Regards,
-- Al

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. VTA ST-120 has 270K Ohm input impedance. I have been thinking about (http://www.elekit.co.jp/en/product/TU-8600) SET 300B amp to run with my preamp and JBLs.

I spent my teens with my brothers L100's and later with a set of my own.  They were party monsters with 100 wpc solid state Pioneer amps.  Great speakers for that but no need for tubes IMO because finesse and detail isn't in their vocabulary anyway.  The new 2018 L100's may be a different story.

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