I have a lot of geezer, tightwad trouble spending over $1k on headphones also. While my go-to headphones are Nectar Hive e-stats, I enjoy the Hifiman HE-560, but that would not be my choice as a complement to the HD6XX. They are bigger and heavier - yes more resolving. There are many here who may disagree (and even most of those may have actually tried them), I find the GRADO RS1e does it much better than the 6XX. Far better air and soundstage.
While “technically” a “geezer” I still have decent hearing, but do not find the GRADO RS1e to be fatiguing or too bright. This is not the case with some of the other Grados, although I do like the GRADO sound. I really like the light weight and the on-ear design. I suspect @Lothar_Wolf and @Wormsdriver might agree. I bought my pair from @MCM who posted these comments in the GRADO thread.