New bike - help needed

Morning folks,
Right, sorry in advance for ramble, but there is a reason. Due to issues with my back and my flexibility (long-standing, and yes I do stretching exercises), I have a fairly relaxed setup on my bike; the stack height is 550mm, plus headset (10mm) and 45mm of spacers(!!), giving an effective total stack height to the stem of 605mm. Reach is 407mm + 110mm stem.

However, I still experience a lot of tension in my neck and shoulders, despite consciously trying to vary my hand position on the bars, relaxing that area when riding, performing off-bike exercises to improve strength and using heat packs and massage to relieve tension. I can't use the drops for very long. These symptoms are after fast club runs of 50-70 miles, where I've put quite a bit of effort in, so they aren't easy rides anyway.

Now, I've tried a shorter stem (100mm) but felt too cramped. I think the stack height needs to be larger, but I can't add any more spacers. (I have actually got another 5mm, but the handling starts to get funny if I add it).

So I'm thinking of at least a new frame. If the deal is right then maybe a bike. If I had thought of this a couple of months ago, I could have snapped up a bargain, but they are a bit thin on the ground now.

NB. I'm 180.5cm tall with an 84cm inside leg, or 5 11" and 33" in old money. I use the bike for fast club runs (maybe 4 hours), 90-minute mid-week blasts, sportives and the odd time trial.

Given the choice I would go straight to my LBS, explain the problem, and check out the options. Indeed, I will be doing this anyway, but realistically I need to save money where I can, so I don't have a firm view on where I'll be buying from. On the other hand, I don't want to just go it alone and end up spending a wad and not solving the problem.

Anyway, I'm currently thinking of:

Cervelo R3 - the 58cm has a stack height of 605, plus 10mm for the headset, so it's 10mm higher than my current position. Suppose I could add a single spacer if necessary (I'd rather not). Also has 11mm shorter reach than my current bike. If this fitted me, it's the one I'd buy - quality, I like the brand/image and it's cheaper than a recent s-works or Domane etc. Frameset about £1700.

Specialised Roubaix 58cm - with a stack height of 631 (including a 10mm headset) it's 26mm taller at the front without adding spacers. I can find a 2011 SL3 frameset from Sigma sport for £1400. Doable, but seems a bit much considering it's a 2011 design. No way am I paying £2k+ for an SL4. Not as keen on the zertz inserts or the big S brand.

Trek Domane 58cm - with a stack height of 621mm including h/set, this is an option. Pricey, though I haven't been able to find a frameset price. Domane 5.2 is £3.1k. No idea what I'll make of the isospeed thingy.

Merida Ride Carbon 95 56cm - 16mm taller than my current position without spacers, although with a reach difference of 29mm, it might be too short. The 59cm is another 24mm taller and 8mm longer. Whole bike with Ultegra can be had for £2k. Not sure of the quality, don't like the looks or image.

Cannondale Synapse - 6mm taller stack height without spacers than my current bike with spacers - cutting it a bit too fine, so probably not an option.

Assuming that a taller stack will solve the problem, I don't know how much taller. If I thought 10mm was going to cover it, the Cervelo would definitely be edging ahead.

Canyon, Wilier GranTurismo, Spesh Tarmac, Trek Madone, Cannondale Supersix are out due to insufficient stack height. I'd rather avoid spacers this time round.

So, help/advice is sought:
1 - Any other suggestions for cause of pain/tension?
2 - Any other suggestions for an appropriate frame/bike?
3 - Any particularly good deals on appropriate frames/bikes including the above list?

If you've read this far, thanks very much!