Did I just make my audio system too sensitive after adding a Herron VTPH-2A preamp?

I forgot to add the cartridge is an AT-ML150/OCC.
Time to figure where this noise is emanating from.

That last detail is telling. AT has their fans but I am not one. Way too hi-fi sounding with exaggerated detail for me. 

You just had a huge, major upgrade. I know. Upgraded to the same Herron last year and even from Audio Research PH3-SE it was a huge redefining experience. You can spend more money and get more features but pretty sure you cannot do any better. And I got Mikey Fremer on my side saying the Herron is as good as ANY Class A component. So relax and don't worry about the Herron.

That cartridge though, the exaggerated attacks it hypes out are no longer being glossed over but instead with the Herron are being amplified in all their AT infused glory. 

What I would suggest you try is first, Keith Herron sends along some loading resistors. Try them. Try them all. Only takes a minute. You may be surprised what a little loading can do for an over zealous MC. At the very least it will give you some new understanding of phono playback.

Next thing I would try, Synergistic Research PHT. These little PHT things have an amazing ability to tweak and tune your cartridge and arm. They come in different "flavors", Green Dream and Black Velvet being my favorites. They stick on easily, are totally removable, and come with a 30 day money back guarantee. They don't eliminate surface noise but they do make a profound and fascinating improvement in what you hear. They work everywhere I've tried them, which will come in handy because....

Last thing I would try is a better cartridge. Koetsu has a reputation for de-emphasizing surface noise. For years I was put off by the price, and not wanting to buy the "cheapest" Koetsu, the Black Goldline, which was about the most I could afford. Once I got it though, what was I thinking??! Totally worth every penny. And then some. Not quite the bargain the Herron is. How often can you buy the best in the world for three grand? Congratulations, you got a lot of stuff you will probably want to be upgrading but the Herron is a keeper for life!

I would talk with Keith as well. I have a handful of loading resistors I'd be happy to let you borrow, just let me know. ^ The VTPH-2A is a keeper for life! 

Gentlemen, as a point of information the AT-ML150/OCC is a moving magnet cartridge, and the 47K input impedance of the Herron’s moving magnet input is correct for it.

Also, I believe that the cartridge has not been manufactured for quite a few years. To the OP, is it possible that the Herron is simply bringing out the effects of stylus wear that was not evident with the Pro-Ject?

Finally, I see that the input capacitance of the Pro-Ject is selectable between 47, 147, 247, and 367 pf. While the input capacitance of the Herron’s moving magnet input is fixed at 100 pf. What setting were you using on the Pro-Ject? If you were using 247 or 367 pf chances are that the difference relative to 100 pf would be resulting in a significant difference in frequency response in the treble region. Although I’m pretty certain that the lower the input capacitance that is seen by that cartridge the better, assuming accurate response is the goal. (Like some other AT MMs I believe the ML150/OCC has a recommended load capacitance in the area of 100 to 200 pf, which represents the sum of the capacitances of the tonearm wiring, the phono cable, and the input capacitance of the phono stage, and in many setups it may not be possible to keep that total so low).

BTW, like the others who have responded I too am a very happy owner of a Herron phono stage, the VTPH-2 (non-A) in my case. I and a number of other members here use it with an Audio Technica AT-ART9 low output moving coil cartridge.

Regards,
-- Al

Slight correction to my previous post:  "247 pf" should have been "267 pf."

Regards,
-- Al