I test with high output humbuckers so that isn't a problem.
I don't really own any brand name speaker. My speaker cab is made by my friend. It is a ceramic, 8 ohm, high efficiency speaker. I believe that it is affecting the output sound by adding clarity. However, I don't think it affects the preamp part. I have tried several audio speakers that I had with me and another ceramic speaker in a cheap practice amp. Similar results.