How to express a loss as a gain in dB?

Not sure you can get 12 db from 6 elts.
From link below, looks like you'll need 8 or so.
https://www.electronics-notes.com/articles/antennas-propagation/yagi-uda-antenna-aerial/gain-directivity.php
Here's a design of one with 6 elts. You can size the elts proportionally to your desired freq. But it has less gain that you are looking for.
https://273k.net/gsm/designing-and-building-a-gsm-antenna/yagi/
If you want really high forward gain, maybe try a corner reflector, which will give you the desired db.
http://www.microwavetools.com/corner_reflector/
You could also try a rhombic, which can have extremely high gain if you have the space for it. There are many articles on rhombic design available.

Power:
2 ohm stereo (per channel): 975W
4 ohm stereo (per channel): 660W
8 ohm stereo (per channel): 400W
4 ohm bridge-mono: 1950W
8 ohm bridge-mono: 1320W

Load Impedance: Safe with all types of loads. Rated for 4 and 8 ohms in Stereo mode, 4 ohms in Bridge-Mono mode

Voltage Gain to 1-kHz, 8-ohm rated output,
CE 1000:
30.5-dB gain at 1.4-volt sensitivity;
26-dB gain at 2.34-volt sensitivity.
CE 2000:
32.1-dB gain at 1.4-volt sensitivity;
26-dB gain at 2.83-volt sensitivity.

Required AC Mains: 50/60 Hz (North American units are 60 Hz only); 100, 120 and 230-240 VAC (+/-10%) units are available.
AC Line Current,
100 Volts: 11.4 A;
120 Volts: 9.5 A;
230-240 Volts: 5.0 A;
At Idle: All amps draw no more than 90 watts.
AC Line Connector: NEMA 5-15P (15 A).

Check the cables. If one lead of an XLR-XLR cable is shorted to ground, 6 DB gain is lost. You haven't really supplied enough information regarding cables and model of the manual mixer for us to help much. Do check that ALL of your audio interconnections are BALANCED lines either XLR to XLR or TRS cables. Grounding one side of a balanced line reduces gain 6 DB. Try swapping mic cables and channels to test where the problem is in the system.

209 lbs.

You should be using Dynamic mics such as a Shure SM58. They go into the first FOUR channels and if there is excessive gain and distortion, push in thee Pad button below the connector for those four channels. Inexpensive consumer microphones used with some Karaoke home systems are not suitable for this unit. You should be using XLR cables on the mics (balanced lines). The Pad buttons reduce the gain by a fixed amount of -20 Db for mics that have too much output to be controlled by the gain controls right above the connector. Ideally those should be no less than 12 o'cloock for good signala to noise ratio. Additional channels beyond the first four do not have as much gain and they do not have the -20 Db pad option.

Bass boost simply adds DB or gain to the bass signal. Some like it, some dont.
The frequency knob is the Low Pass Filter (LPF) that cuts off the amp output above
the set frequency. A bass system usually cuts off below 400 HZ and more often 200HZ.
A good, rattle your car bass note is in the 25-80HZ range or lower but most amps and audio
input devices cant attain a good low note with clarity.
"Low-pass filter allows only frequencies below the crossover point to be amplified. A high-pass filter allows only frequencies above the crossover point to be amplified - useful for keeping low bass away from small speakers, so they can play more efficiently."
Checkout Crutchfields Glossary of Car Amps.
http://www.crutchfield.com/S-LPyoWWgD5qN/learn/learningcenter/car/amplifiers_glossary.html

14g speaker wire is applicable for basically all speakers and setups unless your running over 30' then you should go with 12g.

Your MoBo has (1) of each PCI Express x1 & and PCI Express x16 slots, and (2) standard PCI slots. There are NO AGP bus slots on this board. You'd want to use buy a PCI Express x16 video card instead of a PCI video card due to the better performance offered by the PCI Express cards.

The full details of your motherboard are below:


Product Description The Intel Desktop Board D946GZIS delivers an integrated graphics solution in the Essential Series by combining exceptional value with quality and reliability and supports Intel Core2 Duo, Intel Pentium D, Intel Pentium 4, and Intel Celeron D processors with 1066/800/533 MHz system bus in the LGA775 package. This board features a PCI Express x16 connector, integrated 10/100 LAN, dual-channel DDR2 memory, Intel High Definition Audio (Intel HD Audio) and up to eight USB 2.0 ports.
Processor Model: D946GZIS Chipset Type: Intel 946GZ Express Processor Socket: LGA775 Socket Installed Qty (Max Supported): 0 ( 1 ) BIOS Type: Intel Storage Controler: ATA-100
Memory RAM Installed ( Max ): 0 MB / 4 GB (max) Supported RAM Speed: PC2-4300, PC2-5300 RAM Features: Unbuffered, dual channel memory architecture
Audio Audio Output: Sound card Sound Output Mode: 5.1 channel surround Compliant Standards: High Definition Audio
Expansion / Connectivity Expansion Slot(s): 1 PCI Express x1, 1 PCI Express x16, 1 processor - LGA775 Socket, 2 PCI, 2 memory ( 1.8 V ) - DIMM 240-pin Interfaces: 4 x Hi-Speed USB - 4 pin USB Type A, 1 x serial - RS-232 - 9 pin D-Sub (DB-9), 1 x parallel - IEEE 1284 (EPP/ECP) - 25 pin D-Sub (DB-25), 1 x display / video - VGA - 15 pin HD D-Sub (HD-15), 1 x network - Ethernet 10Base-T/100Base-TX - RJ-45, 1 x audio - line-in - mini-phone stereo 3.5 mm, 1 x audio - line-out - mini-phone stereo 3.5 mm, 1 x microphone - input - mini-phone 3.5 mm, 1 x storage - floppy interface - 34 pin IDC, 1 x keyboard - generic - 6 pin mini-DIN (PS/2 style), 1 x mouse - generic - 6 pin mini-DIN (PS/2 style)

Restore your pc to a previous date. One before you started the update. Click start, click accessories, choose system tools, click restore, choose restore my computer to an earlier time, click next, choose date earlier than update, click next. pc will return to status of that date and should be good to go. Restore will run on it's own, be patient, don't panic. Good luck.

The PGX1 bodypack has a switch on the side to set the gain. Make certain that switch is in the "mic" position. Otherwise, simply turn up the volume on your mixer.