The post's generally not the problem, just the front end. If you have a long headtube then you start getting problems with finding steerers to fit... and so it goes on!
Higher rise stem (say 15 deg if you are using a 0 or 5 deg) with a 1 - 2" riser bar and an extra spacer or two sounds like the most feasible solution - which you're already trying.
On your current bike what it the current drop (saddle height to bar height)?
Something that you may also want to consider is your current flexibility and lower back strength.
If you hamstrings are tight (ie you can't touch your toes) you may benefit from some basic stretching.
There are also a couple of simple back strength excercises (they take about 5 miuntes max per day) which I found fixed all my lower back issues on the bikes. If you want I can send you details of these if you want to give them a try.