Mainly Regency, yes. Heyer began with some set in the mid-18th century, but as she got better at writing and began inserting comedy, she moved to the Regency. Aside from Jane Austen, who wrote it in period, Heyer is considered to have invented the modern genre of Regency Romance. As you say, too many covers with men in knee-breeches and stockings and women in low-cut dresses (some with bustles, some not). Metzger's book covers aren't much better, and of her approximately 41 novels, only about eight or ten have the magic combination of comedy, paranormal or masquerade, and the cosy quality I prefer. Still, that's nearly a dozen books to enjoy, so I don't think I've entirely wasted my time.
Sir Terry advised reading outside one's usual favorite genres, and I agree, if only because it adds to the appreciation of writers who insert allusions and Easter eggs. Investigating romance has also led to my reading criticism by the fans, who have researched the actual history, laws and customs, and point out where the writers stretch and ignore things to allow their plots to happen. Heyer is well-thought-of because she did research carefully as well as learning to be a good writer. (Austen of course didn't need to do the research because she lived then.) From there I've moved to reading books published online of laws of the time, which indicate how horrifyingly draconian the laws were then. E.g.: if a man owned land (thus middle-class at least, not poor) but wasn't rich enough to be allowed to hunt, but owned a hunting dog (defined by breed, not ability or use) and a gun, any richer man could legally confiscate the dog and gun without even having to pay for them. Merely owning a greyhound was considered legal proof of intent to hunt.
I'm currently in the midst of a quite enjoyable book on the Romanov dynasty, but as soon as that's done... 
