Another +1 to add to Chuck H.
The lower frequency roll off is set by the output tube internal impedance rp and the primary inductance of the output tranny. Ultralinear connection reduces that rp by a factor of about 3 times shifting the low frequecy roll off 3 times lower. The problem then becomes core saturation at low frequencies and to address that BIG iron is what you need.
Ian (Cork Sniffer ??)