I have a few Stanwell 9mm pipes and have had some Savinelli 6mm pipes. I've tried just about everything with them. Most adapters are cheaply and poorly made and honestly you're better off without them in there. One exception though. The best filter adapters are Stanwell adapters as they are actually machined to fit very well and are drilled out satisfactorily with a ported airway. They smoke very well without any issues, but they do have one downfall. Gunk does find its way into the fitting area and the stems require disassembly often, with the adapters taken out and everything cleaned with alchohol. I kept noticing that, after normal rest periods, the pipes with adapters still didn't smell clean and ready to smoke but my non filter pipes did. Finally I removed the stems from a couple filter pipes with adapters installed and found that the pipes were fine but the stems were smelly from residue. Stanwell adapters have ridges that succesfully hold them in the stem, which makes them a bear to remove and clean. This whole process has become a bit of a PIA.
I've tried a few different paper filters, which were terrible. Often they change the taste of the smoke and inhibit draw. The best filter I tried was the Savinelli balsa, but in 9mm they are rather pricey. One filter i've not yet tried is the charcoal insert, made by Stanwell and Vauen, I believe.
Simply using no filter or adapter in a filter pipe is acceptable in the few 6mm Savinellis I've owned but in 9mm I've found draw and condensation problems in many pipes.
