I've owned three different ProAc models for the last five years including the 1SCs. Without knowing your taste in music reproduction I'll try and offer what has worked well for me. I have no experience with the Castle Trents, but judging by your results my guess is that the tonal balance of the Trents is very different from the 1SCs.
First, I can't imagine using ProAcs unless I have one or more tube components in the chain. ProAcs don't have the most extended and sparkling highs nor the most extended or defined bass, but they do have a magical midrange and create a holographic image when mated with the right components and set up correctly.
When I bought my first ProAcs, the Tablette 50 Signatures, I thought they sounded bright and never drew me into the music. At this time I was using a Simaudio Moon I-5 and Bel Canto DAC1.1, and with them ahead of the ProAcs the midrange was lean and recessed, and the treble was thin and tizzy. When I changed to a tube DAC and a tube preamp this changed completely. Tubes really seem to transform the midrange and overall balance of ProAcs and also bring life to the soundstage. I've also owned Response 1SCs and had success using various combinations of tube equipment with them as well as with the Response 2.5s I'm currently using.
Specifically, I think the Benchmark may be part of the cause. I'm not making a quality judgement, but I think it may have to do with the tonal balance of the Benchmark. From what I understand the Benchmark has a very neutral and some might say lean balance. I've had great success using the Kora Hermes tube DAC and Audio Mirror non-oversampling solid state DAC with ProAcs. Both those DACs are known for having an analogue-like nature, which I feel compliments ProAc speakers. I'm assuming the Benchmark has better extension than either these DACs, but by contrast has a less pronounced midrange. I just don't think components with a lean balance or a recessed midrange will work well with ProAcs.
I don't have any experience with Arcam equipment so I can't comment on your Alpha 5, maybe others can. Also, depending on the balance of your electronics, using cabling that compiments it is important. Using revealing cables with lean gear is a combination I'd avoid. After using a an arrangement entirely of copper cables, I'm in the process of shifting my cabling to all silver because of the warmer nature of of the tube preamp and tube amp I'm testing.
ProAcs utilize rear ports so it's important to keep them away from rear walls. Depending on the room, a minimum of 3 feet of space behind may be necessary. Watch the side walls as well. If you're too close to a wall it may cause the bass to sound bloated and boomy.
If you can pull your 1SCs out another 1 foot it might help with another problem. ProAcs are very good for nearfield listening. In my room I use a rolling chair so I can change between near and far listening. I notice immediately that in nearfield the whole presentation becomes more intimate, but also the soundstage seems more clearly defined and spaced. You get a better sense of dimensionality when listening near with ProAcs. When I roll my chair back to move back into the audience I get a less forward presentation, but the sense of holography also seems to compress and dimish. When I move back, the bass also becomes more prominent and rounded. Up close the bass has less impact, but it's much more defined. Bringing your speakers out another foot may help reduce the boominess and at the same time help with imaging.
In their set up, ProAc suggests that the speakers be aligned so that the tweeters are on the insides of the speakers and the face of the speaker is pointed directly at the listener. When set up this way, the center image becomes very focused, but width of the soundstage can become narrow. Pointing the speakers straight forward creates a wider soundstage, but the center loses its focus. For my taste, I like my 2.5s set up so that they are half way between straight and pointing at me. For me this is the best of both worlds.
If your sofa is against the rear wall, this can also be contributing to the boominess in the bass as well as some glare in certain parts of the spectrum. Reflections off the rear walls may be the cause of some of the problems. If you can, try experimenting with bringing the sofa out from the wall about 1 foot.
Also look at the trouble spots in your room where reflections might be a problem. Check the speaker's first reflection a few feet in front and to the outside of your speakers. Consider placing sound treatment like Sonex squares, a bookcase or shelves with CDs or books, or possibly a decorative hanging cloth at some point along the side walls or behind you. I'm redoing my listening room right now and with everything off the walls I can't believe how alive the room is and how much it rings and echos. It's clear that I need all the wall cabinets and Sonex panels for best sound.
Besides acoustic or equipment considerations here are some things to think about. What were you trying to gain by purchasing the 1SCs? What was it about the Trents you were trying to improve on? It's possible the Trents just mate better with the equipment you have right now. It's also possible the ProAcs aren't your cup of tea.
The first thing you should address is set up and room acoustics, then after that consider whether you'd like to make an equipment change.
Good luck!
