First, the quality and craftsmanship on the Standards are the exact same as Customs and Signatures. The biggest differences are the flat top, three-piece maple neck (same construction though), and slightly less expensive hardware. The playability and tone are the exact same. Some of the best sounding Roscoes I’ve played and owned were Standards.
Now, for the electronics. IMHO, the OBP-1 is the absolute best preamp to pair with those P/J pickups. They’re already pretty hot and a mid control is just superfluous. Just a bump of bass and slight tone roll off of it gets too harsh are all you’ll need. I’ve owned six Roscoes with swamp ash bodies and trust me, you’ll have plenty of mid.