JCM 900 Surprise

My good friend and I went amp shopping over the weekend.  We were looking at Marshall heads.  We tried a JCM 900 Dual Reverb 100 watt, DSL 100 Watt, MA Series, and a JCM 900 Dual Reverb 50 Watt.  I know the JCM 900's often take a beating on here, but they sounded the best.  We ran straight into them, and then we used a Keeley tubescreamer out front with a DD3 and a chorus in the loop.  The MA Series sounded like a solid state amp, the DSL sounded very good, however, the 900's won out overall.  He bought the 50 watt 900.  He is no novice.  He is over 50 and has been in this game a long time.  Now, some more details.  The 50 watt 900 did not sound very good through the stock Marshall 4x12 cab the store had it hooked up to.  I am not sure of the speakers but I think they were probably the 75's?  None of the heads sounded very good through a 4x12.  Thankfully we took one of our secret weapons, a Marshall 1965A 4x10 cab.  The 900's came to life through this cab.  Also, the 50 watt has a Eurotubes sticker on the back.  Further investigation showed the amp was equipped with 6L6's.  I was very surprised at how good this amp sounded and may take the plunge myself. I think on the Eurotubes website, Bob says the 900 sounds best with 6L6's.  Does this require a mod? Anyway, let me know your thoughts!  What are the downfalls of the 900?  I liked the simplicity, the clean was good, and there was plenty of gain for me.  There was also a lot of love between this amp and the tubescreamer!