Harbeth vs Graham audio

You've actually got Graham, Stirling, Falcon, and the Spendor Classic line.  Good luck trying to audition more than two of those (and Harbeth) in the same system.

I have not heard the Graham speakers but I have heard and owned several pairs of Harbeth and Spendor speakers.  
For my money the Harbeths are better than Spendors.  
My hunch is that it will be hard to beat the Harbeths on sound, and without a doubt the brand equity that Harbeth has will ensure that availability, tech support and resale value will be higher than a fledgling brand.  
If you're able to listen to both and do your own comparisons go with the speaker that moves you the most.  If you're unable to do so I would recommend going with the speaker that you can gather the largest number of positive user impressions as possible.  

Graham make the real BBC monitors in terms of the LS3/5a (similar to P3) and the LS5/9 (similar to M30).  It depends on whether you care about having an actual BBC monitor or the Harbeth versions which to my hears are better for a home based system.  Tube amps or big high end Japanese amps work well for all.  The Classic Spendors are a bit warmer and less open (like my old Rogers speakers) and then you also have ProAc which offers nearly BBC levels of natural midrange but is a more dynamic and open sounding speaker. 

Keep in mind all but the ProAcs were designed for voice and classical/instrumental music primarily.  If you listen to a lot of rock, go with ProAc or one of the more modern Spendor designs. For classical or jazz, they will all sound great. 

I own the Graham Chartwell LS3/5’s. I find them very addictive. I don’t listen at ear bleed levels,and I have a small listening room. I also listen to anything from metal to soft jazz. To my ears,and my tastes,these do everything very well. A disclaimer though,I’ve never had the pleasure of hearing Harbeths. 

I own a pair of Harbeth 30.1's and a pair of Spendor S3/5's. While I like the Harbeths much better over all, male vocals sound much more lifelike to me through the Spendors. I haven't heard any Graham speakers.