I have looping occur under the fabric when I try to sew with a brother XL-2600i

SOURCE: sewing machine XL 5500 settings

Your bobbin is probably too loose. When you set a new bobbin, stop every now and then and try with your fingers nail if it's nice and tight enough. Not extremely tight, but tight, not loose and moves easily when you set your finger between the threads.

With most fabrics (99%) the plate is fine at the the top position, just with very thick and "fragile" (satins etc) you might need to adjust it.

Settings: Tightest tension for the thinnest materials, more loose for the thicker ones, better have a test piece, it rarely goes under 5 anyway.

SOURCE: learn how to change the needle in my brother xl

If, you do not have the manual, please go to Brother.com and download one for free. Also, to change your needle without a screwdriver, use a nickel or dime. Sometimes that works better than a screwdriver and won't strip your screw head.

SOURCE: I want to look at a manual for a brother XL=2600i

Here you go, from the Brother website

http://welcome.solutions.brother.com/bsc/public/us/us/en/doc/manual_index.html?reg=us&c=us&lang=en&prod=hf_xl2600i2610eus&dlid=&flang=English&type2=-1

SOURCE: brother xl-2600I

try changing the needle
try tightening the top tension one number or letter at a time till the stitch look better
take the bobbin out & reinsert the bobbin into the bobbin case, also make sure the bobbin is not damaged
clean under the needle plate & in the bobbin area for loose threads or lint