You could try these in the SEX amp, which version do you have?
I don't generally recommend using Hammond chokes as plate chokes because they are not designed for that implementation. You may find that your high frequency response diminishes considerably using the Hammond chokes. Also, these are going to be much, much larger than the stock iron.
If you have an older SEX amp with the Triad C7X chokes, then it may be worth giving it a shot. If your C7X's have plastic bobbins, this may be a little truer.
I must add a word of caution though - the newest Hammond transformer I looked at had a very crooked stack and the last lamination was not tapped all the way into place.