I have a Lil Dawg Ol Yeller 6g6b clone...4 ohm OT.....I run it at 4 ohms and at 8 ohms all the time....and I CRANK it into an 8 ohm attenuator with no ill effects thus far since 2013. I like the difference in feel an 8 ohm load gives the killer amp. I keep reading both sides, but IME, I’ve never had an issue with an old Fender circuit, with vintage or a clone built to vintage specs, having an issue with a mismatch in either direction. Now Marshall, Vox, Mesa, etc, etc. no way!!! I have heard too many stories about them not handling the mismatch. Someone should invent a little device rated at 8 ohms to plug into the ext speaker jack so when an 8 ohm speaker is also plugged in the amp sees the correct 4 ohm load.