If you are happy with the volume levels that the 250 delivers (i.e., you don't play a lot of bass-heavy music at high volume), then I'd think that the power supply upgrade to the pre would likely make more of a difference.
If, on the other hand, your experience with the 250 into Isobariks is like mine was -- frequent thermal tripping, noticable loss of bass at loud volume, etc. -- then the 135s may contribute more to your overall satisfaction than upgrading the pre.
The 135s do have a different character even at low volume, however. They seem a bit tighter and harder-edged, possibly owing to the increased driver control provided by the greater power and larger power supplies
