From my experience, whenever I have come across people with the same intention as your's, more often than not, I have also got to share their disappointment after spending a lot of money on expensive pre-power combos or even on preamps.
The question a beginner or novice would encounter after experiencing forums like HFV, audiokarma, etc is this very one. After reading up a lot and also after drooling about what others own and talk about, one is often tempted to upgrade or replace.
The fact is, before doing so, you have to have a clear objective (having a question in mind like, will my sound improve if I use a dedicated pre-amp is not a good enough objective). Generally a clear objective is arrived at after a first hand experience, on the type of music you listen to. Hence, rather than wondering whether a preamp can improve your listening experience, get out there and audition power-pre combos, compare what you hear with what you are used to hearing and take a judgmental call. More often than not, when one does this, one would end up taking the right decision.
Remember, pre-amp/poweramp combo space requires investment and there are issues related to matching, etc. Not sure you'll be able to experience a significant difference if you add a phonostage. Phono SQ depends on a lot of factors from quality of vinyl pressing to turntable, cartridge and of course, the phonostage itself. Some integrateds are better than pre-power combos, something I found out after trying out a NAD 3020 vintage amp. Read everything on the subject that you can come across and also listen to as much as you can (audition). Don't open your wallet unless you are investing in atleast a 10-15% improvement over what you have currently
my 2 cents!!
