Thanks, valleyguy. That’s what I was looking for. I’ve pretty much narrowed it down to the HD28 and D55. I play some bluegrass right now, and I’ll probably always play some, but I definitely enjoy playing many other genres. If I was a straight up bluegrass player, I’d probably stick with a Martin. Regarding the D28, I’d have to get an older one because I really don’t like the MLO necks and the nut on the new Martins.
Do you think the 55 can hold its own against the Martin bass wise?
As far as the bass question, the D55 has plenty of bass, I've been satisfied with it, compares certainly to the HD28. Like you I like that bass rumbling in my gut....
For Bluegrass, I've found mahogany guitars work best for leads, though my D50 with an adirondack top, the so-called bluegrass model, holds up well in that regard. That said, plenty of D28s at bluegrass jams, so I think it is all in the hands of the player. You have a D18, so if you can keep that, you'll have a nice balance of a mahogany and rosewood guitar, they will be two very different animals.
I would not be afraid to buy used. Guilds hold up well to use. Made To Be Played, after all. The Tacoma, New Hartford, and Corona models will all sound pretty much the same, and there is little variability between individuals here, and you'll save some money, used Guilds are relatively cheap. You didn't list where you live, but Craigslist and Offerup are good places to start looking locally. That way you can also play before you buy.
Good luck