Sorry, I may have misunderstood your first post where you said you used the "preamp out". I am not familiar with the 5150 and thought that it maybe had a "Preamp Out" and a separate "Effect Loop Send". So I was saying try the Loop Send. I am guessing this is the way you had originally tried it now.
FWIW I have recorded direct ("Loop Send" into Line Level Input of Firepod) with the Cab as the Load and compared it to using a THD Hotplate (set as Load) as the load and used the same Impulse on both and I cannot pick a difference between the two samples. There may be a difference, but my ears can't pick it. Maybe a Hotplate is what you need. I tried the "Hotplate Line Out" and from memory it sounded different than the Amp's Effect Send so I didn't use it again.