Icaudio,
If everything is ok, and you have connected the transformer fore 5:1 configuration you shall have around 15 dB loss from instrument input to XLR balanced out. (no loss, 1:1 from instrument input to monitor amp out)
Because you use a microphone transformer backwards you get "loss" negative gain, and a 1:5 ratio transformer have approx. 15 dB step-up gain, or loss used backwards.
You want loss in a DI-box, because the level from a guitar or bass, (after a 1:1 impedance converting) are to high to match a console microphone input.
I will guess that mostly of active DI-boxes on the market have approx. 5-15 dB loss, and passive have approx. 20-30 dB loss.
If you after all need more gain, use the 2,5:1 configuaration instead for the 5:1 configuraton on the Lundahl LL-1538 transformer and you get only 10 dB loss instead for 15 dB, and have 5 dB more gain out from the DI-box.
(but the best transformer match in this circuit are the 5:1 configuration)
--Bo