I was going to post: . . .
"There are a lot of inexpensive PASSIVE DI units that will do this and they will state so in their instructions. The DI must have a "Link" or "Thru" jack and, if it's designed for general use, a pad of (usually) -40dB. The ART PDB will handle this for under $30 (there is a Nady version for ~$18 and a PylePro version for ~$12), as will countless other PASSIVE DI boxes that have a link/thru and a -40dB (or at least -30dB) pad.
The -40dB pad is set. The amp output is run into the DI input. The link/thru jack is connected to the speaker."
. . . But I think that I should just give Ashcat_lt's post a +1
Paj
8^)