I'm totally biased since I have one for sale, but I'd pick up a used VK-30, then upgrade it at BAT to VK-30SE. If you do so, you'd get 95% of the sound of the VK-50SE (yes I've heard both) at about 60% of the cost. And as a bonus, the VK-30 has both single-ended and balanced inputs and outputs, no adaptors necessary! The VK-50/50SE have only balanced inputs and outputs.
Want to upgrade my Sonic Frontiers Line2
What should I look at to improve on the SFL2? Under $5k used