Out of all your system I'd say that the amp is the strongest/best of the three components. Next would be the CV CT-165s a short nose in front of the Bose bass box.
So, what's good.....
Your Denon amp while rated at 25W/ch will actually deliver quite a lot of power thanks to a beefy transformer and decent capacitors. Put some efficient speakers on the end of them and you'd find that the system would go quite loud and still sound sweet and punchy.
The CT-a65s have some scope for improvement
What's no so good.....
If the amp is struggling then there's a possibility that the big capacitors are buggered. I'd expect one channel to be quieter than the other though as a first sign.
The CT-165's were a budget design. The cabinets are thin, light weight and poorly braced. The woofers have a decent size magnet but the metal framework (the basket) is thin and prone to warping. That affects how well the drivers seal on to the cabinets and the alignment accuracy of the speaker cones. The speakers themselves are a 4 Ohm design.
The Bose bass box (I hate to call it a sub) is really designed to be used with the cubes rather than some other speakers. At the moment you wire from the amp to the bass box, and then from there to the CT-165s, right? So the bass box is acting like a filter (a crossover) and keeping some of the sound signal for itself, then sending the rest to what it thinks is the Bose cubes. They don't handle much bass at all. The sub is designed to work with the cubes and send only the frequencies that the cubes can accept. The CT-165s can handle a bigger audio range, but the bass box can't be adjusted to accomodate them.
What to do....
Long story short, a better pair of floor standing speakers and ditch the Bose bass box.
There's plenty of classic floor standers from Mission, Ruark (very good spkrs and yet under valued), Monitor Audio, B&W, Castle, KEF, Maudant Short, Tannoy. These are all good British makes.
If you're not too fussy about finish then there's a pair of Monitor Audio Monitor 14 speakers on Ebay right now at an insanely low £21.00 with just 19 hours to go.
Short-term measures....
A little TLC with the CT-165s probably wouldn't go amiss. Two things to try are 1) adding some ballast to the bottom of the speaker, and 2) making sure the woofer frames are flat and securely screwed to the cabinet.
Ballast means weight. In this case, some kiln dried sand (no moisture) then bagged up so it doesn't spill and then inserted in to the space at the bottom of each cabinet. You have to remove the bass driver to do this, but that's okay because you're going to check out the shape and then screw it back tight anyway.
Adding ballast will help keep the speaker stable and allow it a better chance to produce crisper bass. A couple of kilos of ballast in the base of each speaker should help.
At this stage I'd try wiring the amp direct to the 165s.
