You can make improvements by focusing on building from the inside out but I think your most significant improvement will be different minimonitors with larger woofer (see list below.)
Keep the VTL and switch out 2 x 12AU7, 2 x 12AT7, 4 x EL34 tubes for more power and different sonic signature. Then start your auditions of speakers with...
Helicon 300MKII
Reference 3A
Totem Mani 2 Signature
12type tubes Mullards
EL34 types from Mullard, Genelex Gold Lion or Black Sable
Upgrade your cable and IC to get the last 10% to 15%
Keep the VTL and switch out 2 x 12AU7, 2 x 12AT7, 4 x EL34 tubes for more power and different sonic signature. Then start your auditions of speakers with...
Helicon 300MKII
Reference 3A
Totem Mani 2 Signature
12type tubes Mullards
EL34 types from Mullard, Genelex Gold Lion or Black Sable
Upgrade your cable and IC to get the last 10% to 15%