Other ideas:
A Rounder Records Bluerass Sampler.

Seldom Scene. Old Train is one of my favorites with some fine singing.

JD Crowe and the New South. I especially liked the lineup with Scaggs,Rice and Jerry Douglas.

There are tons of antholgies out there where you can hear Bill Monroe, Flatt and Scruggs, the Stanley Brothers the Osborne Brothers, Jim and Jesse,Don Reno and Bill Harrell, Mac Wiseman etc. doing a lot of the old standards.

A fun one, although more bluegrassy than Bluegrass and a kind of rough around the edges live album, is Old and In the Way. Done in the early '70's with Jerry Garcia, David Grisman,Peter Rowan,and Vassar Clements features a few old standards and a bunch of contemporary songs done in the bluegrass style. I think it may still be the top selling Bluegrass album of all time which probably causes the hardcore some dyspepsia.