That's a $64k question Bob. Always look for the low hanging fruit before making system component changes. But here are some suggestions:
IC/Cables - For more details upgrade your cables. For example, look into purchasing upper mid level IC (hopefully balanced XRLs) cables in the $800 - $1500 range. And you can upgrade power cables for 90 and source. Nice change with great value.
Tubes and Fuses - Look at upgrading tubes and fuses to enhance the clarity and body. More powerful KT88/6550 ($200 to $300) and 12 types and 6SN7.While not a big expense upgraded fuses also have a small impact on the sound...mainly clarity.
M-150 Monoblocks - Switch to monos to improve full height, width, and depth, and overall musicality. More significant change to system with the added cost. You could also upgrade the tubes for monos as well.
Loudspeakers - Maybe you want something totally different like oversized bookshelf AudioNote HE/SE or Quad. That way you keep your 90 and focus on upgrading speakers, ICs, and tubes for pre and power.
IC/Cables - For more details upgrade your cables. For example, look into purchasing upper mid level IC (hopefully balanced XRLs) cables in the $800 - $1500 range. And you can upgrade power cables for 90 and source. Nice change with great value.
Tubes and Fuses - Look at upgrading tubes and fuses to enhance the clarity and body. More powerful KT88/6550 ($200 to $300) and 12 types and 6SN7.While not a big expense upgraded fuses also have a small impact on the sound...mainly clarity.
M-150 Monoblocks - Switch to monos to improve full height, width, and depth, and overall musicality. More significant change to system with the added cost. You could also upgrade the tubes for monos as well.
Loudspeakers - Maybe you want something totally different like oversized bookshelf AudioNote HE/SE or Quad. That way you keep your 90 and focus on upgrading speakers, ICs, and tubes for pre and power.