I had the Tonelabs SE for a while before i sold it. If you like more tube like response, the tonelabs is much nicer but the high gain sounds off it are not fantastic where as the the POD XT excels in that section and it has a lot of functions than the tonelabs SE. Cant comment on the GT8 never tried.
But i would just buy a good 2nd hand Tube amp and build up my pedalboard... which incidentally is what im doing now.
For multi fx you can try acoustic amps also, they also tend to be flat response.