Matty, I often find myself repeating this, but most of us tend to mix OUR playing too loud on top of the arranger's outputs. This is a natural tendency, wanting to hear yourself well, to make sure your timing is good, and you can concentrate on YOUR parts, but in a studio, once everything is laid down (with the musician able to hear their performance loud and clear), it is then MIXED so that everything is balanced and nothing dominates too much.
The trouble with an arranger is, there is no way to hear what YOU are playing loud and clear, and still have the main outputs be more balanced. The only way around this is, I'm afraid, getting used to playing and hearing YOURSELF at that lower, 'mixed' volume.
The way to do this is, first of all, listen to records, CD's whatever, in styles similar to what you are shooting for. Then record yourself playing the song, and then compare it to the record. Be brutal! Be critical! Don't worry too much about your playing (plenty of time later for that!), but concentrate mostly on the balances between parts. If you find, after recording, that you have somewhat buried the backing, back down the volume of the keyboard Parts, save the Performance, and re-record. Keep doing this until you think that the balance is back to closer to the record mix.
NOW... you need to get used to this, now lower, balance between you and the backing. It takes a while, but once you do, you will sound very polished and professional. It is quite strange - arranger players are almost the ONLY musicians that have to get used to hearing themselves (their parts rather than the accompaniment) lower than anyone else. The price we pay for playing along with the main mix, rather than having stage monitors to bump up OUR parts.
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An arranger is just a tool. What matters is what you build with it..!