Morning Ernie,
Since you asked, I am going to try my best to express my opinions on these 2 units.
Also, please keep in mind that I am nowhere near a wordsmith like Phil.

First off, the PB101 will give you a more modern tube sound while the P2688 tends to lean a little over to the vintage arena.
The PB101 will need to be matched with better tubes, with the exception of the 101Ds since they are used solely for voltage regulation, in order to truly show its potential.
Stock tubes is just that, decent but much to be desired.
We have several customers who wanted to roll tubes right away but I asked not to do such until at least they can establish an acoustic baseline then once the better tubes are in...that's when they can realize how good the PB101 truly is.
Concerning the P2688, it is truly a remarkable Preamp and the beauty of it is, IF you prefer, you won't need to roll any tubes BUT the rectifier.
When I was at Phil's place to listen to both of his PB101 & P2688, he rolled a few rectifier tubes in and they only differed slightly until he put in the Shuguang 274B and then...WTH, both of us looked at each other and smiled. What it means is the 274B is the choice for P2688.
I know this may have left much to be desired as a 'sales' pitch but writing is not my main forte.
All I can add is the P2688 takes all of the PB101's strong SQ to another level effortlessly.
FWIW, I have a PB101 in my bedroom and a P2688 in my reference setup.

Hugh, you sell them both, what's your opinion about the differences between these two superb preamps?