There's a huge contrast between designing gear in a one-off fashion, either for personal use or for an individual client, and on the other hand designing a piece of gear for production and sales to a larger public. Both types of design undertakings have legitimacy, but the divergence in approach, priorities, considerations is worthy of consideration.
In one-off design, one has the freedom to use rare and/or limited supply components (such as vintage audio transformers, obscure NOS tubes, etc), unusual materials, and unorthodox construction (often labor intensive) techniques. It can be a lot of fun, and gratifying to build something singular and unique.
On the other hand...
When designing for production, a whole slew of additional imperatives and considerations emerge: The longer-term availability of individual parts ( and avoiding the need of parts which can only obtained via one or two vendors ); the crucial importance of efficiency/ease of assembly in the overall design (time really is money!) ; the repeatability/ stability factor of having a design which "wants to work correctly" and not requiring tweaking, critical individual parts selection etc; reasonable failure-proofing so foreseeable minor owner/user foibles/ mistakes won't result in melt-downs or explosions ; the importance of rationalizing/harmonizing the BOM of a design with other products being offered: How many parts can be selected (resistors, capacitors, nuts, bolts, other hardware etc) from existing in-house supplies so as to keep the parts inventories as streamlined as possible?; Aesthetic congruity: designing the unit to aesthetically meld with one's chosen "house style" without clashing; Can it profitably be build at a price which is comparable (or ideally, competitive) to similar products on the market?; And yes, one must meet all of the requisite safety, environmental and other regulatory standards too ---whew!
So yeah, to design a competitive, successful audio product which is reliable and safe, which actually makes money and builds the manufacturer's reputation in the market place -- there's a whole lot of real work involved beyond just designing a circuit which sounds awesome.
Likely much of the above will seem obvious to many. Hopefully not too pedantic -- just submitted as a little food for thought ; )