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

Tubes every 6 months? I've had power amp tubes last for years!

Depends on the amp and the reliability of the tubes in some amps they can last over one year or more.What about when tubes blow out this happens.

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.