People That Have Upgraded From Harbeth 30.1s....

The sound of 30.1 is just more refined. It handles delicate things better. Bass is less but it is punchy. The 30.1 tweeter is a tough to beat. I'm considering trying at home a set of Raidho C1.1s that my dealer has.

This was my experience too. The m30.1 tweeter endows it with a more sophisticated and complex sound than the C7ES3. In my 20x14' room I found the presentations similar, but the m30.1 was more lucid and less warm. I much preferred the later.
Some of the differences between these two may also be down to cabinet construction - the m30.1's thicker, braced, cabinet was obviously a bit different to the C7. A simple rap test showed this.

Upgrading from the 30.1s is a challenge due to the lack of criticism regarding Harbeth speakers in general.

Some of the above comments suggest that the bigger models in the range will give you more of the same as to what the M30.1s will give you. Alan Shaw has stated that he had tried to get the SLH5+ to sound a little more open than its predecessor (SLH5). 

However, these improvements seem to be of an incremental nature rather than anything dramatic. Heck! I've even read reviews where the diminutive P3ESRs  have been compared to the mighty M40s.

There really does seem to be a family sound between the models where even the M models (monitor series) share an affinity with the rest of the line. I guess  it's natural enough given that all models share the same material for the main driver and that the cabinets also share a similar construction design.

So it looks like it's down to the SLH5s or if funds allow, the M40.1s. I've read that they work really well for nearfield listening. Or just stay with the M30/1s which share the same tweeter as the M40.1s. Here's a great write up on the M40.2s. 

https://www.whatsbestforum.com/threads/harbeth-monitor-40-2.25222/

I'd be very surprised if the original Quads were an upgrade. They certainly didn't impress in my system. A totally different kind of sound, comfortable with even mids, but somehow sounded challenged at both frequency extremes.

A perspective on the quality of amplifier issue...I bought my C7s while living in England for a year. I connected them up to my roommates inexpensive Yamaha integrated amp (around 50wpc) which was connected to a equally unimpressive CD player. The room was excellent--just the right size with plenty of room to move the speakers away from walls. The ceiling had multiple angles as it was the top floor of a three story house. I was really impressed with the sound we got. Was it GREAT sound? No. But what impressed me was that even with modest electronics the Harbeth's did NOTHING terribly wrong. When I brought them back to the states after my tour was over I connected them up to my all Naim system. Did they sound better? Yes, a lot better. My takeaway is that the C7s are simply a very forgiving design, one that will deliver better performance as you upgrade the rest of your system but which will sound very good while you are finding the money to improve the upstream components. This may be one reason why Harbeth is so popular.

I can echo the experience of tobes.  I replaced 7es3 with ATC 19v2.  In my system, and based on my listening preferences, the ATCs work much better.  Which speaker is best?  That is not for me to say.

I have settled down with Harbeths. They don’t have the best dynamics and deepest bass, but overall they sound just right, musical and lovely. They don’t need great electronics too, sound good even on few hundreds $ amps. But if you pair them with something like pass labs - they will sing. I have had about two dozens of speakers from different manufactures and heard even more.