I always like to hear a piece in my system before I decide to buy it. But if you buy a Ref 150se used at a good price. You can sell it and not take much of a loss if you don't like it.
ARC has had some amazing pieces and some that were just so so. The Classic series and later V series were ground breaking. My Classic 60 was just so much better than my D70 it was amazing. But after the V series there were no real stand outs until The Ref 110. With the possible exception of the VT130. I had a VT100 mk3 for a number of years. Originally I was going to sell my V70 but found I really just preferred the V70. The VT100 did a lot of things better but it lacked presence. The VT100 lacked that rightness. It was not until the Ref 110 did I feel I could finally sell my V70. The Ref 110 has the best attributes of both. I think the Ref 150se does also. But what matters is how it sounds to you in your system.
If you buy a new amp I still wouldn't sell the Classic 120s if it were me. I wish I would have kept my V70.