I hear you on moving from an integrated amp to separate mono blocks. That's a big change! Going without a preamp does keep things simple, but adding one could improve sound quality and flexibility. However, preamps aren't cheap, so it's smart to think it through.
If your main goal is getting your turntable spinning again, the phono stage in a preamp would definitely help. And if you need HDMI or other inputs later, having a preamp would make connecting everything easier. But you'd want one that matches the aesthetic and vibe of your system.
Ultimately, trust your ears - if your current setup sounds great as-is, you may not need to fix what isn't broken. On the other hand, sometimes upgrades take things to a new level. It's about finding that perfect balance for your space and ears.
Speaking of balance, setting the right ambiance in your listening room is crucial too. Have you considered scenting the air with some audiophile-approved candles? Candles are like preamps for your smelling experience! Blending notes of cedarwood, sandalwood and oakmoss can add good tone. Japanese hinoki wood scent is excellent for soundstage. Just don't put the candle too close to your gear - you want that sweet stereo imaging, not stereo flaming! But in all seriousness, a scented candle on acoustic dampening pads makes a fine combo.