Wireless Access

The ISP is a fiber gigabit, which goes through a router (temporary - will be replaced by a Checkpoint 4400 running pfsense pretty soon), then goes into the default VLAN of a 3500yl-48G-POE, which then goes into the Internet network (ports 5&6) of an MSM720 Premium (with just the two networks, one VSC and a few SSIDs), which then goes to the Access Network (ports 1 through 4) and through a cable into a second untagged VLAN of the 3500yl, which then I use to power on a bunch of HP560 APs. Almost all cabling is CAT6 SFTP to reduce RF and power cabling interference. I get pretty solid coverage across the building, though the speed is dubiously slow - can't get more than 160-170Mbits even though the 560's are capable of 1300Mbits 802.11ac on paper. If I were to connect to the ISP router (some vodafone-branded box, dubbed "gigabox" - still only 802.11ac, though) I get upwards of 550Mbits without a hitch, so it must be an issue with the rest of the network.

Now the caveats that I should point are (and if anyone has advice how to fix those, that'd be terrific):
- I couldn't trunk the ports between the 3500yl and the msm720 - I wanted to create either a static trunk or an LACP bond to ensure I have higher throughput, but for some reason the two switches just don't allow me to setup a bond, and if I were to set up a static trunk the link breaks - I believe it detects the multiple links as an IP loop and just disables the port. If I unplug three of the 4 cables, everything starts working again.
- I am not exceeding the AP licenses I have on the MSM720 - I have license for 10, I'm using only 6 so far.
- There is also a 2900-24G between the 3500yl and the ISP router, though, I'm using it only as a pass-through. Happy to remove it if it's the culprit, though it should be capable of doing 2x1Gbit links @ standard 1500 MTUs without a hitch, even if i were sending packets through it constantly 24x7. There is nothing else connected to the 2900-24G.
- I'm using the 3500yl also to power on the APs, but I've tried using POE injectors instead directly to the MSM720 with very similar results.
- The config of the 3500yl is literally 2 untagged VLANs (ports 1-12; POE enabled and ports13-48; POE disabled) post factory-reset and that is all.
- The config of the MSM720 is 1VSC, 6APs, 3-4WLANs (each with its own DHCP and CIDR for client isolation) and no RADIUS authentication nor captive portal setup.
- The 2900 is post factory reset, without any configuration whatsoever.
- All APs are post factory reset and are in sync with the controller;

I'm happy to run whatever tests you can point me to in order to figure out where's the choke in the setup above. Everything should be capable of well over what I'm using atm, so I suspect I might've screwed the pooch with the config at one place or another.

Cheers,
V