Using the information that we have in the pictures, namely that a standard desktop sized enclosure will fit between the stand you can deduce a bit about the size of the support platform at the top. The desktop series are 6-1/4" x 6-1/4" so I'm going to guess you've got about an 8" square contact patch at the top to rest the speaker on. Sounds small at first until you remember that it both allows for bolting the speaker down and also is coated with the Bumpon material which will provide more traction for larger speakers at angles.
The Harbeth's, for reference, are 12" x 7.5" (roughly) and I'm sure they'll sit comfortably on the stands.
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SB3/Arcam CD73 -> Hertsens Signature DAC -> Project Menace -> HD650 | Woody HF-1
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