If we're talking about the same Workingman's 300, I believe its only 160W at 8ohms. If you add another 8ohm cabinet, it will boost the wattage to 270W at 4ohms, but will sound louder due to the increase in speaker surface area.
That head is supposedly good to go all the way to 2.7ohms (340W).
+1
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"SPEAKER JACKS: Use an unshielded 2-conductor cable (NOT A GUITAR CORD!) to connect any speakers to your Workingman's 300. Use a high quality, heavy gauge cable of at least 18 gauge (the lower the gauge, the heavier the cable). The total impedance of the speakers should be 2.6 Ohms or greater. This means you can use 1- 4 ohm cabinet in conjunction with 1- 8 ohm, 3- 8 ohm enclosures or 6- 16 ohm cabinets. Recommended speaker cabs from SWR are: Goliath III, Workingman's 2x10T or 4x10T, and Big Ben 18" (you can use any three of these as they are all 8 ohms). You may use one of the following: Henry 8x8, Triad I and Goliath Jr.III (4 ohm) in conjunction with one 8 ohm enclosure. Power Output: 160 Watts @ 8 Ohms, 270 Watts @ 4 Ohms, 350 Watts @ 2.6 Ohm"
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"Expanded-Design" Tuned, Ported Cabinet
Custom Hartke 5" Aluminum-Cone High-Frequency Driver
Four Hartke XL 10" Aluminum-Cone Bass Drivers
Power Handling: 400 Watts @ 8 Ohms
Frequency Response: 30 Hz To 12 kHz
Sensitivity: 99 dB @ 1 W/1 M
Voice Coil: 1.5"
Magnet Weight: 60 Oz.
Parallel 1/4" Inputs
Dimensions: 27"(H) X 24"(W) X 18"(D)
Weight: 99 lb.