With the 6 ohm speaker, LM1875 is 53% efficient, which is the portion that goes to the speaker, with the remainder of 47% to warm the heatsink. The sum (53%+47%) is 100% of the power input. I didn't count power used for warming the cables, bridge rectifier, transformer, caps, resistors, etc. . . but hey I'm just estimating here.
TI assigns a 34W total power budget to LM1875, and that is 34W budget shared/split between speaker and heatsink. In their example, 8 ohm speaker, 60% efficiency is then 20W to the speaker AND 14W to warm the heatsink.
The sum is 34. The 8 ohm speaker gets 20W
I think it could do slightly more, so I'll be using 36W total. But, the amp is not 100% efficient, so we'll have to multiply the efficiency by the total power budget. With the amp efficiency at 53% while driving your 6 ohm speaker, just multiply 36*0.53 to see that there will be 19W available to the speaker and 17W to warm the heatsink.
The sum is 36. That 6 ohm speaker gets 19W
That will take approximately 21+21vdc rails, at most.
VA for transformer is not exactly the same as watts (even though both are volts*amps=watts), but the difference is a minor 10% or so (power factor?).
If 6 ohm speaker, it is possible to use up to 36va~40va 15+15vac transformer for each monobloc without excessive risk of exploding the amplifier.
Those are maximized figures.
Exceeding that, could be sketchy and result in reduced longevity.
Pushing LM1875 to max requires current limited supplies most sensibly done with monoblocs.
P.S.
If you wanted a much more convenient stereo build, go get the 12.5+12.5vac 2A center tap transformer at the nearby radio shack. After power board (as easy as one bridge rectifier and a pair of big caps), that's 17+17vdc rails. The results from that option are 13W to most speakers and the excellent convenience that the datasheet sample schematic is optimal at that voltage.