Any downsides to transformer volume control preamp

I have been investigating transformer volume controlled passive preamps from Silver Rock and Bent Audio. They appear to be very transparent from user comments. I was wondering about the downsides of using a transformer for volume control. Are there any? I would probably replace my Placette passive if there was a noticeable improvement. Thanks for any advice in advance.
i also want to research! what is the price on these units?
Since I have no direct experience w/xfrmr volume controls, I cannot comment. However, I, like you, have been interested, but have not taken the leap. FYI, Absolute Sound did a review of the Silver Rock a couple or so years ago (along w/a lower priced isolation xfrmr w/o volume control) and the reviewer thought it was indeed superb. As I recall, he found little to fault other than the price---$3K+. The reviewer was definitely impressed and wrote glowingly of the advantages.

Good luck.
I have owned three Silver Rocks. The first was a prototype with a two plane ground...two ground posts in the back. It caused a hum in my system, but may have been damaged in shipping. I received a replacement from Audio Consulting that was the latest Granito metal version with the four plane ground. This unit did not hum, and made the background blacker, making previously unheard detail on CDs clearly audible. It is also grainless and totally uncolored. If you think you own a great active pre-amp...the Silver Rock will change your mind. I owned a Wyetech Opal and many others, and the Silver Rock is far better for transparency, attack and decay, lack of grain, and natural tonality. You also don't have the lackluster dynamics of regular passives. No downside that I can see except for price and the looks. I now own the wood Signature version, and it's even better than the metal version. If you come across one, buy it. Antique Sound Labs also makes an Autoformer passive that I have owned for under 300. Good for the bucks, but it's no Silver Rock.