Well I was on 472kHz WSPR RX overnight with the preamp in circuit so at good sensitivity. Sadly, not a single new station received, although (maybe?) the S/N of those received was a little better. To be honest, with QSB it was hard to be sure. So, in future I may use the preamp at times although I am pretty sure I have copied most of the active stations that my current system is going to copy.
UPDATE 1014z: Since before breakfast I went QRT on 630m (472kHz) WSPR. I am now active on 10m and 6m WSPR again until this evening.