Spendor D9.2 Run In

That's normal.  It takes awhile for everything to break in and settle down. I've got three D series with LPZ tweeter.  Absolutely amazing when broken in, but give it a couple hundred hours (no kidding).  They sound bright when new but neutral and glorious in the end.  Different sound from traditional Spendor, the D series is

When I have my ear right up on the speaker, the brashness seems to be coming from the midrange and not the LPZ tweeter.  The speakers do throw the mids and highs forward, which I like - especially the tweeter.  I can hear it at a distance.  What I find odd is voices seem to be higher in pitch than I am accustomed too, and that's the midrange.  I am hearing detail I never heard before, which startles sometimes.  But it's the pitch, and at higher volumes especially, brashness or tinnyness, that is a bit unnerving.  I will try to be patient.

I wonder if you possibly have bad tubes in your MC275.

Spendor has made some duds with respect to tweeters but has never failed to get the midrange right. You might try to borrow a SS integrated from a friend just to rule out the amplification. 
 

The D series speakers measure nearly ruled-flat and as a result will sound “lean” compared to many/most competitors. 

Make sure your source material is good.

internet radio etc can vary a lot