Many OTL tube amps are definitely quite a bit higher than 32 ohms out. Grados actually have a very very flat impedance curve (compare RS1/SR225 to Sennheiser HD600/650/800 on headphone.com), which means that the losses due to a low damping factor (less than 1 in this case) won't necessarily cause the usual frequency response aberrations you'd expect with more typical speakers & headphones. In that sense, Grados actually seem to be quite "tube friendly". As always, the answer to anything nontrivial just "depends" on the specific context in question, and the oversimplifications we often use to dole out recommendations can allow some otherwise nice match-ups to fall through the cracks.
The main consequence here will be a loss of power delivery from amp to headphones, as most of the amp's output power will be dissipated as heat before it hits the headphones - and the actual ratio dissipated vs. delivered will depend on the actual output impedance. I suppose my concern here would be more about the headphones' effect on the amp, rather than the other way around.
Wow man, I know there are some real sweet sounding low-powered tube integrateds out there, but 8 Watts on 89dB/Watt speakers just would not cut it for my listening habits. Again - different contexts => different answers