The amplifier is direct coupled at the output, and yet the speakers barely move. Why? The solid-state amplifier probably has an idle current of only 50 mA. A trivially small amount of current compared to what it can put out at peaks.  Let's put it in perspective. 50 mA against the speaker's 8 ohms only equals 0.4 volts, about one third what a D cell battery used to test the speaker's polarity puts out. Considering that a Dynaco Stereo 70 also has an idle current of 50 mA per output tube, which is effectively amplified by the winding ratio of the output transformer (23:1) to over 1 amp; the solid-state amplifier is running a very lean Class AB indeed.

Futterman and Circlotron types. The end we hope to design into the circuit is to disable the opposing active device automatically, whether it be solid-state or vacuum-state, when only one power supply rail fuse blows. If both devices or banks of devices shut off at once, the speaker or very expensive headphones cannot be damaged.
   Let's stat with a very simple hybrid amplifier. The tube provides voltage gain and the MOSFET provides current gain. The bottom N-channel MOSFET defines simple current source for loading the top MOSFET, which works as a source follower. (Yes, this output stage is single-ended, Class A sonic gold.) The Op-Amp makes up the center of a DC servo to keep the DC offset to a minimum. 

    If the power amplifier is run in heavy Class A operation, then the potential speaker damage greatly increases. A Class A amplifier capable of putting out 100 RMS watts must deliver 5 amp peaks into an 8 ohm load, which means that its idle current must equal at least half the peak current. 2.5 amps of current into an 8 ohm load equals 20 DC volts and 25 RMS watts of power. This is not music watts we are dealing with here, but finger-frying-speaker coil burning watts. The usual answer is to use active protection circuitry and a relay at the output to protect the speaker. There techniques are both useful and laudable, but they can be augmented or even eliminated by careful design.
   Lest any readers lose interest because the word "tube" has not used for one paragraph, do not despair, the design techniques we will cover apply also to tube solid-state hybrid amplifiers as well as pure tube OTL amplifiers, both 

    As configured above, the amplifier will damage the speaker when the top fuse blows. The bottom current source will still conduct all its preset current right into the speaker's voice coil. The amplifier will also damage the speaker when only the bottom fuse blows. The top MOSFET will lose its correcting bias voltage from the Op-Amplifier, as the Op-Amp will lose its negative power supply connection. The MOSFET's source will follow the Op-Amp's climbing output voltage and take the speaker's voice coil with it. If both fuses blow simultaneously, then the speaker is safe, as nether MOSFET well be energized.