Speaker or amplifer

That explains a lot. The REL has an 8” driver and only goes down to 30Hz at -6dB so is more like a woofer than a subwoofer, which explains why you’re not getting much slam. All you probably need is something like a Rhythmik F12G to get you what you’re missing.

https://www.rythmikaudio.com/F12G.html

What sub are you using?  It’s a little curious that you’re lacking slam while using a sub assuming it’s properly set up. 

In looking a little further I found these specs from the Sonetto v g2 Stereophile review…

The Sonus faber loudspeaker's voltage sensitivity is specified as 89dB/2.83V/m; my B-weighted estimate was 88.6dB(B), which is within experimental error of the specification. Sonus faber specifies the Sonetto V G2's nominal impedance as 4 ohms. The impedance magnitude (fig.1, solid trace), measured with Dayton Audio's DATS V2 system, remains above 4 ohms for most of the audioband, with a minimum value of 2.9 ohms between 90Hz and 100Hz. The electrical phase angle (fig.1, dotted trace) is high at low and very high frequencies, which means that the effective resistance, or EPDR (footnote 1), drops below 3 ohms between 43Hz and 46Hz, from 65Hz to 203Hz, and above 4.3kHz. The minimum EPDR values are 1.95 ohms at 34Hz, 1.26 ohms at 81Hz, and 2 ohms from 6.5kHz to 7.7kHz. The Sonetto V G2 is a very current-hungry load for amplifiers.

In general these are not speakers I’d drive with a 100Wpc tube amp.  Also, while they’ll get you down to 35Hz vs 42Hz with your current speakers and that will definitely be an improvement it’s still not all that low.  So again if you already like the sound of what you have now, if the Hegel doesn’t do it for you I’d be inclined to add a couple subs rather than getting the Sonetto v g2 as your amp may struggle with them, and the bass, imaging, and soundstage improvements with subs will very likely surpass what you’ll get with the Sonetto v g2.  Just my $0.02 FWIW, and best of luck.

If you try the Hegel and you find you still prefer the overall sound of the Rogue the best thing to do would be to get two subwoofers that will improve bass, dynamics, as well as imaging and 3D soundstage.  Best of luck. 

What are Sonus Faber iii?  Olympica?  What specific improvements are you looking for, and what’s the rest of the equipment in your system?