To the best of my knowledge, there is no Topping DAC that can output the required voltage (3Vrms) via RCA output. No, it does not have to be reached. I just can't figure out why you designed an amp whose max output cannot be reached (at least when pairing with most DACs (including all topping DACs).
The reason is that the headphone amplifier has to be able to deliver high current at low voltage into low-impedance loads, and high voltage at low current into high-impedance loads. And it needs variable gain (or input attenuation) to work comfortably with headphones of different voltage sensitivity. But it will rarely, or more likely never, be used with a headphone with an impedance and sensitivity that requires both maximum voltage and maximum current from the amplifier at the same time.
Since output power is current times voltage, the maximum power rating of the amplifier is a function of its current and voltage ratings, but is otherwise a relatively meaningless number. I made this chart based on the L30 power/impedance specs. The "Power", "Current", and "Voltage" numbers are peak values, and the 'Vin" numbers are RMS.
Topping says the L30 can deliver 26V pk-pk, which corresponds to 13V peak as given in the "Voltage" column on my chart. With a 300 Ohm load, we see the amplifier is voltage-limited to this value. At 32 Ohms, the available voltage has reduced to 24.2V pk-pk due to internal resistance inside the amplifier. At 16 Ohms, it has either run into a 660mA current limit, or more likely, the available voltage has sagged to 21.2V pk-pk.
The L30 product page has an unusually complete set of specifications, which the calculations confirm. There is enough information here for most headphone owners to determine whether or not the L30 will meet his or her needs. Or, if they're not comfortable with math, they can just read a review and confidently conclude, "Yes, the L30 makes enough power to drive just about any headphone to deafening volume."
Your complaint seems to be that the E30 cannot drive the L30 to maximum output power, to which I respond, "So what?" But we can calculate how much power the L30 produces into these loads given a 2.0 VRMS input signal at maximum gain/volume. The values are 2.22W@16 Ohms, 1.12W@32 Ohms, and 119mW@300 Ohms.
The question a potential buyer should be asking is whether the L30 will play loud enough with his or her headphones, given the headphones' impedance and sensitivity. The Sennheiser HD6xx has a rated impedance of 300 Ohms and a sensitivity of 103dB@1VRMS. The E30 driving the L30 driving the HD6xx will produce a SPL of 118.5dB. Is that enough for you? No? Go buy a DAC with 3.07VRMS output and you can deafen yourself by an extra margin of 3.7dB SPL.
I'm not a Topping fanboy at all, but I don't like trolls.