contacted the folks at BOSE about repairing the 901's, their reply was to sell me an updated series VI speaker with new EQ for close to $800.00 with trade-in of my series IV. Repair kits are less than $100.00. They also said starting midway with the series V speakers they switched to using cloth surrounds, should last much longer.
The dust caps cover the voice coil and keep the dust/dirt out. Once these caps are removed you can then add thin plastic shims in the voice coil gap to keep the speaker cone centered while the glue sets up. This way you avoid the possibility of the voice coil rubbing after the glue has dried. Then you have to remove the shims and glue new dust caps on the cone. This procedure makes sense to me but for 18 speakers this is a lot of extra time/effort. I guess you had good luck without all the shimming.