Is there a difference? Yes. However, different is not always better. If you 'love the sound' I'd advise you to leave well enough alone. Enjoy the music.
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As you noted, the preamp preamplifies the signal before sending it to the amp. It also allows switching between sources, the inclusion of a phono stage, may provide a balance control, another set of preamp out outputs, etc. Any of these functions can also be included in an integrated amp but sometimes manufacturers save some of these functions for their separates.
I think it's safe to say that manufacturers build their more expensive separates to sound better than a less expensive integrated. However, in a given system the improved sound quality of the separates may not be large enough to justify the higher price or may not even be able to be heard at all.
I'm with jmcgrogan, if you love the sound of your current system stick with it until you don't love it anymore. Many people spend a lot of money and don't end up loving their system.
Separates offer greater flexibility if one tends to change components often. Also, separates offer the promise of "better" sound because the amo and pre amp dont need to share the same power supply. Im not a tech guy, but those are the 2 main reasons separates are nice. However, your Rowland is very high quality and you would probably have to spend signif more to get "better" sound thru separates.
Since you're not using phono, the preamp section is probably attenuating the signal instead of amplifying it, which is why passives work for CD sources.
Without the need to equalize and amplify phono cartridges, the role of separate preamps might be to just increase the number of chess pieces to create the desired complexity to become obsessed with the chessboard in putting together the magic checkmate system. You miss out on that if everything gets integrated. It can save you some money, but a big part of the fascination in audio is the number of pieces and the vast array of choices in combination. Just add the "everything matters" assumption and you're in for years of calculations, research, and nervosa.