Yes, thank you. Jye replied to my email. Jye confirmed that the specified gains are linear.
I was interested because I am doing a "paper exercise" to compare expected behaviour of the NuPrime STA-6 with my former amp, a Yamaha AX-570. The Yamaha amp has a (power) gain of 29 dB, and this worked well with my Roland Digital Piano + JBL speaker combination. I recently sold the Yamaha amp, and have been looking for a suitable replacement. The JBL speakers are ancient, but I still like them.
I have since had another look at my math and I have realised a curious oddity: 28x linear (voltage) gain is approximately equal to 29 dB (power) gain. Ie. the "numbers" work out to be "about the same".
Therefore, I have decided to order the NuPrime STA-6. Destination is Canada.