I'll weigh in because I've been playing a SR370EF for 8 months, and I've been looking at all the upgrade options ever since I picked that bass up and discovered I love fretless. I love the SR370EF, it is an excellent fretless, only limited by it's super thin neck and somewhat meh preamp.
Here's what I concluded in search for upgrade path:
1. ESP LTD is an excellent value and has a solid sound, is a good player, but is heavy and the ergonomics of the body are not great. It is thick and tough on the plucking arm.
2. Ibanez SRF700 is very good, but the Bart p'ups are a little meh. The side lines are adequate to indicate position on the board but it has an extended range and they aren't going to be super helpful in the 24 - 30 fret line area, but then again, lines don't help much in upper register anyway.
3. SRF500 series is pure piezo and will lack EB tone, but is a great player.
4. SRF900 is excellent, superior to the 700, better p'ups. I almost bought one instead of the Sadowsky MetroExpress I just ordered. It has ghost lines, which you are not really going to be able to see while playing. You would orient by the side dots, but I had concern as to how visible those are in daylight, since they are luminous variety.
5. Sire fretless have limitations right now, either with visibility of side lines, preamps (or lack thereof), p'up issues, or limited availability. Once they get their issues worked out, they are (or will be) solid choices.
Anyway, I went with unlined with the Sadowsky. You can get a lined one pretty close to your price budget from SW.
