Herron VTPH-2A vs Rogers PA-2 Phono Preamplifier

the Herron is problematic to roll for better sound.

That's an odd way of putting it. The Herron comes with such high quality tubes it is hard to find better. Yes. Much better. Carry on.

I suspect the Herron might respond less obviously to tube rolling because it uses an FET (transistor) in its MC gain stage. Thankfully, transistors are hard soldered in place. Otherwise folks would be transistor rolling. Is there some reason to believe the Rogers unit would be competitive with the highly regarded Herron? Why would you use tube rolling as a criterion by which to judge? Real improvements to be obtained by tube rolling, unless the replaced tube is worn out or defective, are very limited and subjective only. Usually the practice does not raise one piece above another if it was inferior to begin with. In my opinion.

It's not that the Herron's tubes are so "high quality"; in fact they are among the cheapest you can buy on the market.  Keith did make sure that the batch of these cheap tubes that he uses are quiet.  The Herron is voiced for the EH tubes - by design.  Keith designed the pre so that you can easily replace with other cheap EH tubes when the need arises - a smart design.  However, if you don't like the "voice" you should look to another phono pre.  Other phono preamps can produce different sonics when you roll the tubes - whether they are better is debatable and a matter of personal taste.

I know nothing about the Rogers unit, so can't help you there. 

Frankly, the VTPH-2A sounds so good with the stock tubes I've never given any thought to opening the case to even bother to see what tubes are in it.

And this is from a guy who has hundreds and hundreds of dollars of tubes that I've tried in my last preamp. 

By the way, I have also not looked inside the Herron VTSP-360 ESP that I replaced that tube preamp with either.  It's also that good.

Also because if you’re using an LOMC cartridge, you’re basically listening to an FET. If I understand the Herron phono correctly.