Man what a tough crowd. The SP6B is too rolled off, is smeared and doesn’t image all that well. You must like some crispy sound. I would venture that it is not a function of the Pre, but rather the speakers being employed. The Arc sound and my SP6B experience was almost the diametric opposite. The ARC reminded me more of solid and very detailed, state not a warm, romantic piece, with midrange bloom, you are describing. You could try Telefunken 12AX7s, and see if that firms thing up a bit. For me it didn't matter.
I don’t really use my SP6B that much because it was too sterile, went to a 6SN7 based amp. Mapletree and Atmasphere make this kind of pre, but they aren’t going to yield you more detail.