Thanks for the replies. Strange, isn't it, how machines vary? I asked the question because the dealer seemed to be suggesting that people are damaging their machines by using these thicker threads. But knowing the quality of Superior threads, I find it hard to believe that they would label a thread as machine quilting thread if it wasn't suitable for that purpose on most machines. In an earlier conversation with the same dealer, he more or less said I should use fine thread. Janome themselves implied the same thing: their customer service department recommended Robison Anson embroidery threads for quilting. It's certainly easier to keep the very temperamental tension on this machine correct if I use those or similar. But I don't always want to be that limited - and now, as I've described in another thread on here, my machine is having problems with Masterpiece as well, which definitely isn't thick!
I'm going to post my question on the Janome forum and see if I get any responses there.