The amp likes low inductance..
So bigger dia tubing the better (there is a point of low return though..ie dont be silly and use 3 inch tubing.)
To improve more then use 2 or 3 loops in parallel, spaced say 6inches apart.
Or even better use 4 crossed parallel loops..
To improve hf over lf use smaller dia loops..above pic shows loops of 80cm ( circa) squares..i have the preamp switchable directions, hence why 8 loops in pic.
Above works well from dc to 20mhz..