SF Extremas and Bat vk50se

.
I have the Extremas paired with a Supratek Cortese pre-amp (www.supratek.biz) and an old Mark Levinson 23.5 Amp. It is a heavenly match up and I am quite delighted with the results.
.

The 50se to 51se is a monster upgrade. The 51SE is in a different league. Much much better bass and treble extension, definition and detail. Queiter, better depth width, decay and wieght on all the notes.

Your nuts not to do this upgrade. Search, several other posts here on this. Absolute sound has a review also which compares the two.