Carver used to make a device called the Z1 "Z Coupler" that is exactly what you describe. It has high and low level speaker inputs and RCA outs. You can find them from time to time on eBay. There are also car audio companies that make a similar device. The Carver is a small box for home use.
It basically does the same thing a sub's speaker level input does. I used one a long time ago and it worked well given the shortcomings of the system. I'm sure it wasn't the bottleneck but the system it was used in wasn't very revealing so it's hard to say. Figure $50 each for a new variant or used Carver Z Coupler, it may be better to get a used pre-pro or new receiver.
A forum community dedicated to home theater owners and enthusiasts. Come join the discussion about home audio/video, TVs, projectors, screens, receivers, speakers, projects, DIY’s, product reviews, accessories, classifieds, and more!