Subwoofer for Ayre AX-7e?

HSU ULS-15 MK2 or Rythmik F12/F12G.

For sure better for the money than REL.

Though I am biased toward Vandersteen, I have to second Johnny's recommendation.
Your speakers are probably the weakest part of your system, and the M5 are the lower end of the Epos offerings. 
That being said, the Ayre integrated that you have is a very good unit and if paired with Vandersteen 1's would probably knock you out of your seat.
And, the price for a used pair is very reasonable.

I also am using the AX-7e in my office with a pair of Zu Omen Bookshelf speakers and HSU VTF-1 subs.
The Ayre handles them at line level with no problem.
When funds come available, I am going to get a pair of Vandersteen VLR's to replace the Zu's.
B

I also am using the AX-7e in my office with a pair of Zu Omen Bookshelf speakers and HSU VTF-1 subs.
The Ayre handles them at line level with no problem.

Hi Bob,

Doesn’t that mean you have to adjust the volume control on the sub whenever you change the volume of the signals provided to the main speakers, to keep them at the proper level relative to each other?

Best regards,
-- Al

I’ve owned your integrated amp. It’s awsome with a pair of Vandersteen 1’s. The bass is satisfying in most small to mid sized rooms like I assume you have. I base that on your system.  I’d go give them a listen for sure. 

Thanks for all of the feedback and suggestions. It’s a fair point that the M5s are the weak link. They’ve served me just fine in the mid/upper freqs for the last 12 years and so I’ve really wanted to fill out the low end in a way that wasn’t going to amount to new speakers. But the Vandersteens sound like a serious top-to-bottom upgrade so perhaps I need to approach it more holistically. I just don’t want another set of speakers, however amazing, that still leave me wanting substantially more bass. I don’t have new mains AND a sub in the bank account right now.