I don't know where you got your specs from but from the PSB website, the Imagine C has the follow specs as shown below. Please note the bolded sections if the nominal impedance. The center channel will not dip below 4ohms so 4 ohms will always be its worst case impedance. The Onkyo will drive it but you will be limited on loudness. If you plan on playing it quite loud, you will likely end up driving this receiver into clipping and risk damaging the tweeters. I would add an external 3 channel amp to drive the left/right mains and the center channel. I would look to Emotiva as they sell decntly powerful amps for more than reasonable prices.
Imagine C Center Specifications
Frequency Range
On Axis @ 0° ±1.5dB 45-20,000Hz
On Axis @ 0° ±3dB 47-23,000Hz
Off Axis @ 30° ±1.5dB 45-10,000Hz
Lf Cutoff -10dB 40Hz
Sensitivity
1w (2.83V) @ 1m, IEC-filtered Pink Noise, C-weighted
Anechoic Chamber 87dB
Typical Listening Room 89dB
Impedence
Nominal 4 Ohms
Minimum 4 Ohms
Input Power
RMS, Clipping < 10% of the Time
Recommended 20-150 Watts
Program 150 Watts
Acoustic Design
Tweeter (Nominal) 1" (25mm)
Titanium Dome with Ferrofluid
Woofer (Nominal) 2 x 5 1/4" (135mm) Woofers
Clay/Ceramic filled Polypropylene Cone,
Rubber Surround
Crossover 1,800HZ, LR4
Internal Volume Design Type .54 cu ft (15.4 liter)
Bass Reflex
Size (W x H x D)
Millimeters 521mm x 191mm x 346mm
Inches 20 1/2" x 7 1/2” x 13 5/8”
Net Weight
Kilograms 2.2 kg/each
Pounds 26.8 lb /each
Shipping Weight
Kilograms 13.9 kg/each
Pounds 30.5 lb /each