Cartridge or Pre Amp

Assuming the cartridge you are referring to is the AT150MLX, as shown at the link you provided, if you are not presently using the lowest input capacitance setting of the Tube Box DS (47 pf) that is the first thing I would try.

Also, how long is your phono cable? If it is particularly long its capacitance could be adversely affecting sonics.

The AT150MLX has a load capacitance recommendation of 100 to 200 pf, which is a low value, and anecdotal reports I’ve seen here confirm that too much capacitance will adversely affect its sonics. That number represents the sum of the input capacitance of the phono stage and the capacitance of all of the wiring between the cartridge and the phono stage.

Good luck. Regards,
-- Al

@bpoletti the Herron is $3650 and it must be very good at this price, but the OP using $399 Pro-ject tube phono. The JLTi cost $750 including shipping worldwide and it's much cheaper than previous $2k version. 

@joysofsound The AT-ML150 OCC or modern AT150MLX ? 
Anyway, both are very good MM cartridges, no worries, but the AT-ML150 is better. 

Upgrading your cheap tube phono stage is what you need. 
The JLTi is the best for both MM or MC for reasonable price and the latest version is much cheaper than previous swiss version, but the performance is even better.  

   

@chakster And the upgrade cycle will be never-ending.  Spending a bit more up front will end future money spend on continuing the mediocre performance of cheap electronics.  Using a Herron VTPH-2a will certainly make the AT150MLX sing up to its potential rather than limiting it with the cheap #%*@ you suggest.

Maybe you’re right, but based on my experience it’s hard to motivate a person to jump from $300 product to $3000 product.

The JLTi is not cheap sounding phono stage, if you know who was Allen Wright, who designed JLTi with Joe, and if you will look at the price tag for the first 3 Wright’s versions you will notice they are over $2k. New version does not have that fancy design, but amzing for the money, now $750 (AU$990) including shipping (extrenal power supply is optional upgrade).

Luckily there is NO tubes at all, which is a limited factor in my opinion !

For MM cartridges the JLTi is one of the best phono stage on the market at very reasonable price, no other phono stage alows the user to change the loading for MM cartridges.

@

I listen to Herron Audio equipment and wonder why they are not priced in the $30,000+ range.