I too think the transformer may be the reason. I don't know exactly all that the 18/36W switch on my Ceria does, but regardless of volume the difference in low end is really very noticeable, much more prominent than the volume difference.

When I swapped the transformer in my Valve Jr. for a beefier one, that too gave it a whole different low end response and overall it sounded much clearer with the breakup happening only later, here's my video on it:



Also serves as a good example how EMG85's always sound with EL84 amps, more or less...thru a 1x10" Marshall speaker here. My favorite response type :-)

Furthermore, the 120W Ampeg I had, it had so much clean bass end I could easily produce more low end than our bassist...and it had transformer big as a child's head, making the amph weigh like hell...and the reason I sold it. No casters even!