I have one, or rather a clone. Very clean and very loud, starts to break up at about 5 (halfway up the volume) which is very loud. In fact, too loud for most bandmembers. So you'll need a good OD pedal (I use the Fulldrive II myself ). I love this amp. It's perfect for Blues and Rock but you'll need an OD pedal. It takes pedals very well.
I'm not an absolute authority on this but the 1974x is rather different from the Bluesbreaker. It has 2 EL84 output tubes as opposed to the 4 KT66 in the Bluesbreaker, for example. I think Marshall copied the circuit of the Watkins Dominator when they first came out with the 1974x (sometime around '67 or '68?). So it's not quite correct to say the 1974x is the little brother of the Bluesbreaker -- an assumption made perhaps due to the almost identical cosmetics.
Anyway, you can't go wrong with either amp. However, I'd go for a good clone rather than a Marshall reissue. The clones are handwired, have the correct cab size and the correct tubes, none of which you have with Marshall's Bluesbreaker reissue.