It really depends on which marshall cab you have i guess, seems like a fair trade considering price wise, you might be able to make a 100 to 150 more if you sold your set up, but is it worth the hassle.
But at the same time you are not really down grading, and trust me that tremoverb combo is one heavy beast. If you truly want to down grade look for a 50 watt head and a 2x12. But just so you know, both the tremoverb and the dual rectifier you can pull two tubes and run saftly in 50 watt mode as long as you match the ohms up, more on that if you are interested later. But honestly not realy a big difference between running a 100 watt head into a 2x12 compared to running 100 watt head into a 4x12 when it comes to volume level and apartment playing. However tone will change considering you are going from a 4x12 into a open back 2x12 most of the time and you might not like it.
Now to tone wise, if you never played a tremoverb just be prepared that it is more bright with more upper mids, different tone then what you might be use too, but you might just love it. ALso like i said above, going from a closed back 4x12 to a open back 2x12 is a huge difference and you may not like it.