Neglecting room effects, it can be calculated that at a 10 foot listening distance 6 watts into two 89 db 8 ohm speakers will result in an SPL (volume) of 90 db. That will be loud enough for most listeners with most recordings, but it will certainly not be loud enough for many listeners with recordings having particularly wide dynamic range (such as many classical symphonic recordings).
Although many and probably most pop and rock recordings are compressed to dynamic ranges (the difference in volume between the loudest and the softest notes) of less than 10 db, lots of music in other genres can have dynamic ranges of 30, 40, or even 50 db. A 30 db dynamic range means that 1,000 times as much power is required for the loudest notes as for the softest notes. A 40 db dynamic range means that 10,000 times as much power is required for the loudest notes as for the softest notes.
Also, I would by no means count on the 6 watt SET as being able to sound its best when putting out close to that number of watts.
Finally, I note that the **minimum** power recommended at the Merlin site for your speakers, for a tube amplifier, is 17 watts. Unless essentially all of your listening is to dynamically compressed recordings at moderate or low volumes, why "upgrade" your amp to something that is likely to disappoint on at least a few (and perhaps many) of the recordings you listen to?
IMO. Regards,
-- Al
