Definitely involve him in this decision. I guarantee he already has some strong opinions on the mater and having to fake smile for weeks after receiving a gift that ticks none of the boxes is soul crushing for both the giftee and the reciever. Buy it together. He may have saved up some cash to sweeten the pot a little, you both could learn something about bass amps, and, well, you're doing it together.
Years ago, my now deceased wife tried to surprise me with a gift. I'd been grousing around about wanting to replace my keyboard that had been stolen. I was thinking a Roland or Korg workstation of some sort. Not knowing anything about keyboards, she relied on the music store guy for help and ended up with a clearance/non-returnable Casio toy, the day of my birthday party. I made until about four hours after the party, before I had to tell her. She was very upset with her ungrateful bastard of a husband for a couple of days, despite my best attempts at tactfulness. But I took her with me when I went shopping for the performance keyboard I was looking for and then she understood. Avoid that whole last scenario and read the first paragraph again. Best of luck!
Added: Make sure, before you even get there, on the amount you have available for this gift. But don't buy something just to buy something. As we say in Pro-audio, the wrong equipment at the right price, is still the wrong equipment.
