Cartridge or Pre Amp

Your AT-ML150 is a great cartridge! Do not change it to MC or whatever MM, it will be the biggest mistake. The AT-ML150 has gold plated Beryllium Cantilever and Micro Line stylus. Beryllium is not available anymore from any manufacturers. MicroLine stylus is one of the best stylus ever made! You have absolutely amazing cartridge and it’s hard to beat by MC or MM, only if you will upgrade to AT-ML170 or AT-ML180.

The weak point is your TUBE phono stage, look for JLTi phono stage made in Australia, because some nice NOS tubes will cost you as much as the new phono stage (lol). Personally i would avoid cheap tube phono stages.  

JLTi + AT-ML150 is a killer combination, it is possible to change the loading up to 100k Ohm (instead of 47k Ohm) if you will ask Joe to made a free mods to his phono stage, he could replace internal resistors to 500k Ohm, after this mod JLTi is the most flexible phono stage with RCA plug load resistors on the back. MM or MC can be loaded to whatever value.

Get a Herron Audio VTPH-2a and never have to worry about phono stage performance.  It is as good as it gets and can handle high-output and low-output cartridges with equal aginity.  And the price for such a stellar performer is competitive with even mediocre performers.

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.