Re: What is the positive and negative sign next to the channel means?
As a temporary test, try setting your default channel width to 20MHz channels under Wireless > Radio Settings, then go back to my.meraki.com and refresh the page and see if those plus and minus signs go away. Then go back to 40MHz and they should return.
This should be indicating multiple bonded channels when you're running 40 or 80MHz channel widths, so 161- indicates a 40MHz channel bonding of 157+161, while 149+ indicates channels 149 and 153 bonded to be a 40MHz channel. Try it both ways and check the results.
I think you'll find that the channel reported is the primary channel (like 149 and 161 in your case) while the positive or negative offset indicates the direction up or down the spectrum for the secondary channel (which would be 153 and 157 respectively in your case).