AC1900 C7000v2 - Slow WiFi speed

I have a AC1900 WiFI Model C7000V2 Combo Modem\Router and getting poor download\upload speeds  Comcast is coming out this pm to "troubleshoot" the problem after of course going through the reset of router and signal refresh, checking Mac address is registered etc etc.  Been through this many times with Comcast and I've configured many modems\routers over the years not a newbie to how this should work. Its been slow and getting worse for the past 2 months.  Looking for suggestions on what else to investigate as the cause of the slow speed?  I have replaced the cable from wall to router with a brand new one.  The wifi amplifier is in basement where cable enters house and is the original one that comcast replaced about 3-4 yrs ago when I had a similar "signal problem".  Cables to my house are underground and to my knowledge are original to the house from 2004.

I have Comcast Internet on my bill listed as "Blast!® Internet Plan, Download speeds: up to 400 Mbps, Upload speeds: up to 10 Mbps"

Via Comcast speed test I am getting 63.8 mbps Download and .4 mbps upload.  I realize being hardwired I should expect to get the advertised speed but via WiFi I understand it will be slower but not expecting it to be this slow. 

The modem is in a bedroom on my second floor about 30ft from bedroom where I am running a iMac Quad core i5 3.4ghz

The speed is about the same when connecting to the 2.4 or 5g signal from the router.

WiFi Spec on the Mac is:

Card Type: AirPort Extreme (0x14E4, 0x16E)

Supported PHY Modes: 802.11 a/b/g/n/ac
Supported Channels: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 144, 149, 153, 157, 161, 165

Thanks for suggestions and awaiting the comcast answer of "yeah its your modem, its old technology etc"  however, its only 3.5 yrs old kind of expect it to last at least another 2 years before its truly "out of date"