I retrofitted my Beseler 23CII XL condenser head with a Dichro S diffuser. It came with only one mixing chamber that worked for formats from 35 mm to 6x9 mm. However, I lost three full stops of exposure (some exposures were over 2 minutes), so I purchased an after market "light multiplier" on e-Bay. B&H sells it, too. It works for 35 mm (i.e., I got back my three stops and then some), but nothing larger (I use 6x6 format). If you work primarily with 35 mm, purchasing the multiplier even at B&H's price of $45 (plus shipping) is worth it. I'm trying to figure out how I can configure the original mixing chamber so it will work for 6x6 without a significant loss of light. In the meantime, I have a Beseler 45S for MF & LF.
If you decide to buy a second-hand light multiplier, be sure both pieces are included. I've seen e-Bay auctions that only include the mixing chamber and not the metal column.