Since I have been using my Mark IV as a test bed for speakers (not intended), I have a speaker that sounds killer but does not quite fit with the rear mounting method (Magnet is too large). I already had all of the 8 typical mounting holes drilled into the baffle. My favorite speaker so far is the Fane Studio. More recently I installed WGS Black Hawk HP 100 which fit well as rear mounted speaker but it lacked the power handling of the Mark IV at higher volume settings (started to smell like it overheated when volume set to 3). Awesome speaker in all respects except for power handling. I wanted to reinstall the Fane Speaker but also wanted more room for tube replacement. Went to the hardware store and got a 1.5 inch drum sanding kit for use on a power drill. Since I have air tools and compressor, I made use of them for better control to prevent removing too much material. Worked like a charm. Installed the T-nuts on the back side and popped the speaker in. The next issue, clearance for the grill. I removed the 4 screws used as stand-offs and insterted 4 thin nuts onto the screws in reinstalled them. Also had a rubber foam matt that was 1/4 inch thick, cut in thin strips for grill support, attached with double sided tape. Used 2 inch stainless star head self drilling deck screws along with decorative washer. Looks great. Grill is easily removed without having to take the chassis apart. Since I have converted to front mounting of the speaker, I was able to get rid of the chassis rattle I was getting with the rear mounted speaker. This rattle has been there since I bought the amp. I always thought it was tube rattle, nope. The Fane speaker never sounded better when mounted this way. Many have claimed that front mounted speakers sound better than rear mounted speakers, since I have had the same speaker mounted both ways, I would agree.