I haven’t found the SLB300 to be sterile. Sounds way better than a BG direct to headphone by far.
Seems like a pretty hi tech piece of gear but I question the need for amp emulation because the SLB doesn’t benefit from BG effects and a BG amp scooping if your goal is to make it sound like a DB. Think of it this way, they optimized the sound so that it sounds as natural through a flat response system so that would be the ideal starting point. Flat response PA’s give me way more flexibility in terms of tailoring the tone and compensating for a room’s acoustics.
I do have a set of open back headphones and it gives me more of an open sound. I also have a cheapo set of cheapo Behringer headphones that came for free with a mic that I purchased which I use a lot because they are very slim and lightweight. I do HPF because these headphones are not flat and are bass boosted, but otherwise this set of $15 headphones have worked well for me.
The results will also vary based on what you’re playing and expecting to hear. When I’m playing with a backing track it’s fine direct. If it’s arco and I’m playing solo it can sound a little too direct. Maybe add some reverb if you want to emulate a larger hall, which these days is an easy effect to add by many other means as well such as a plug-in or a microverb.
When I practice through headphones, I do have the blend knob all the way to mic, which is not how I normally would do it if through speakers.
I guess what I’m saying it might be worth not spending on headphones until you’ve spent some time practicing because you might find that it works very well as is.