Integrated Question

Many recordings are deficient in bass response. Don't be so hasty in blaming the Modwright amp for this! You can experiment with the bass roll off upper frequency on the REL subs.

"Just want to shore up the bass." - add another pair of REL subs.

Get the Stereophile Test CD and use the bass tones to evaluate the placement of the REL subs in your room. They may not be in an optimal position.

Your listening spot may not be optimal either!

Rooms have positive and negative nodes  where bass notes are increased or decreased in level. This is elementary acoustics! Your listening spot can be in one of those nodes. Use the Stereophile Test CD playing bass tones while you walk around the room. You may have to rearrange all four speakers and listening spot to achieve an acceptable result.

Looks like I am the only one here discussing room nodes for bass. Such ignorance! Sure, go buy another amp - add more watts! LOL! Go buy some wire for use as a tone control! LOL! 

The flattest bass response in a room is having the woofers placed at the boundaries of two surfaces, i.e. the walls and floor. Putting woofers away from surfaces yields the most uneven response. Roy Allison discovered this decades ago and used it to produce a line of speakers with exceptional bass response.